As a debian user whos current primary desktop runs ubuntu. Speed wise, it seems that ubuntu loads and enables everything by default. Where debian tends to only do what you tell it to do, after being told to do it.

Maybe this has been addressed as well, but ubuntu is very window manager specific. When I first installed ubuntu I tried using the IceWM manager. I always use it, why wouldn't I? Except that doing so in that 10.04 version of ubuntu, caused everything to break. Even the ability to use a usb pen drive. No amount of old school modprobing of usb-storage and other manual steps could work around that in my experience at that time. Switch back to the default ubuntu wm and it's simply peachy.

Eventually I tweaked my ubuntu install to be more of an xubuntu install running XFCE. Having at least a CPU meter and NET meter "IN" the taskbar is an absolute must for me in a window manager. It allows me to see issues instantly. Instead of waiting five minutes and going this doesn't seem to be doing anything, let me check on it. Debian in my opinion just has a lot less bloat. And fewer running applications means more RAM and and CPU cycles for other intended efforts.

I think that's exactly the heart of the matter
(unneeded, autostarting services, "just in case" seems to be the Ubuntu Way)

Only just seen this thread now due to Verdaz having posted in it. Anyway to make a point that others seem to have missed the only real way to makea valid comparison between Debian and Ubuntu is to grab a Stable version of each that are comparible. Get Debian Squeeze and Ubuntu 10.04 and run them side by side. Once Wheezy comes out get it and run it side by side with 12.04 and see what one is faster for you.

Why is Ubuntu slower, some aspects have been pointed out but thngs like "ureadahead" and "Plymouth" really buggered up the start process from 11.04 on for many users. Then, as already mentioned, we have PulseAudio which the developer himself said Ubuntu stuffed up its implementation, jockey which runs the entire time and other poorly thought out aspects which are great if you like bling but a pain in the butt if you want a reliable and fast system.

So in answer to your question its the Ubuntu bits of the system that make it slower than Debian.

This is exactly the answer... this is why usually about every 12-18 months, a really good Ubuntu release will come out. At this point, they've typically properly "implemented" some new features, while new features are still very early in development and haven't had a chance to have a negative effect yet..

10.04 -- hideous. I had lots of problems here
11.04 -- Not a bad release in my opinion.
11.10 -- not to bad
12.04 -- Very good in my opinion. I actually used Debian on my desktops and Ubuntu on my laptops for quite some time. After using 12.04 for a while, I went ahead and put Ubuntu back on my desktops. It took a while for Unity to grow on me, but I like it better than Gnome 3.
12.10 -- I consider it almost unusuable. Constant unity crashes, etc. just got frustrated so I went back to 12.04, where everything works fine.

As for speed... I've not noticed a HUGE difference in speed. Actually, I think Ubuntu 12.04 boots a little faster, but there are some things Debian may do a tad faster. Honestly though, it's tit for tat.

10.04 LTS was put out with out following Ubuntu rules for LTS releases. No package should be used in an LTS that has not been used in a previous release. The LTS is supposed to be Stable.

Plymouth, poorly implimented, was default in 10.04. Plymouth does not, to this day, behave well on my hardware. I can boot every distro and version that uses Plymouth except 10.04. With it I can't boot a Live CD. If I install it I can't boot it. Makes my hard drive sound like a rock crusher at work.

With 12.04 they had learned their lesson. Unity testing actually started in a serious manner although not as a default install with 10.10-testing.

If you think about the rule on no unused packages going into the LTS you will also see that the odds are that the x.10 after an LTS is going to get a lot of packages that were in the hands of the devs but didn't make it to the previous LTS. The x.10 before and LTS gets a LOT of packages that may be almost ready dropped into it so that they will qualify for the LTS. The x.04 between LTS releases is the most likely to be pretty good in between the LTS releases.

LTS releases are based on Debian testing (currently Wheezy). This is to promote the possibility of stability.

The other releases are based on Debian Sid (always unstable). This is to get those cutting edge packages.

All releases after a LTS release are really just steps in the path of building the next LTS.

10.04 has worked very well for my wife on her laptop. Squeeze (10.04 was built from it when it was testing) works well on my desktop although I use Wheezy or Sid more (Sid right at this momment).

Wheezy, on which 12.04 is based, is not considered stable by Debian yet. Will probably be released sometime in February depending on outstanding bugs. Debian uses bug rules to determine if a release is Stable or not. It is getting there but not quite ready yet.

I didn't get into Ubuntu until 8.04.1. From what I read it was pretty bad when released. Was great when I installed it. Lenny, on which it was built, I installed about 6 weeks after installing 8.04 and both were rock solid.

Squeeze will be supported for a year after Wheezy is released. The,roughly, 2 year dev cycle for Debian is what has driven the 3 year support for Ubuntu LTS releases. The new 5 year support will be interesting to watch. Should work out pretty well as Ubuntu has always supported the LTS server editions for 5 years but it will be hell on the devs that maintain the repos.

I liked 8.04, it was much better than 7.10 which I rarely used and I even went back to 7.04 to be able to use my wireless because ndiswrapper in 7.10 would not function at all for me. At one stage I used ndiswrapper from 7.04 in 7.10 and it worked perfectly but 7.10 had other issues as well. I used Lenny briefly after using LMDE which was Squeeze at the time. Lenny was far more stable than 8.04 even though 8.04 was rock solid for me. Apart from a brief foray with 7.10 I used each version of Ubuntu from 7.04 on till 11.10 and ugraded at least one of my machines on the day the new version was released. I found 10.04 to be ok but comparing it to Squeeze there is no contest Squeeze wins hands down. I do have 12.04 in a VM but Unity just annoys me and always has. I have it just so I can keep up with Ubuntu and be able to help Ubuntu users who are lost.

I now move between Wheezy and Sid (in Cobber of course), using Sid 75% of the time. I mainly use MATE and Gnome 3 (Shell). Mate is nice and light, something Unity cannot claim, and Gnome 3 is really quite functional also something Unity cannot claim in my instance anyway.

I just dumped ubuntu (10.4 ugraded to 11.10) since I had > 2GB of updates pending and felt it was time. I'm back on debian and back on sid. I've been toying with bootable usb flash drives and SDHC cards with readers. And that's pretty sweet just to have that ability. The hard drive on the old laptop failed, so it now boots via an 8GB cruzer stick. And I'm back to using that laptop as a bridge to leech it's wireless device and avoid buying more hardware. Like I used to do before it's hard drive failed.

Back to the comparison I guess. In glxgears I get about 5.4k fps in debian squeeze. But in debian sid I'm only getting 4.8k (which is worse than I was getting in ubuntu). To be fair I've enabled pulseaudio because the openjdk seems to be compiled around pulse, which solves a number of audio quirks with the game I play most often. Without the need to package my own openjdk with a non-pulse orientation. And my ati video card is now deprecated so I'm using the 12.6 driver supplied by ati, as sid doesn't have that driver packaged up yet. It is available in experimental, but I don't really want to mix and match. So I went old school as the 12.6 version fails to create debs for sid. I'm not sure if it's that or pulse that is responsible for the fps difference, or other things.

I can get upto 5.2k fps in glxgears in sid if I stop a number of things, like pulse audio, but it's not consistently 5.2k. So far it looks like hotplugging a network device is on by default (udev / net.agent + sleep), and rpciod, nfsiod modules in kernel which I don't have a need for atm. If I need to tranfer files between computers on a network, I just run proftpd until it's transfered and then stop running proftpd. Or just use a USB flash drive, or cloud service. And there's no way I'll be hotswapping a PCI card while the machine is running for network related stuff. I'll probably attempt a custom kernel for the first time in a few years just to get rid of, or allow the option to NOT use those network related things I don't actively use. I'm not entirely sure why they're defaults atm. Maybe for some form of cell phone I guess, but my phone is a phone and powered only by a power adapter, not usb. These young pups with their bells and whistles.