I'm having a strange behaviour with Slackware 10.1 and sata drives used with software raid. If I boot with a 10.0 cd and create a raid volume, the system begin to resync it with a 'normal speed' (more than 30MB/sec). If I make the same thing with a 10.1 cd, the resync is much slower, less than 3MB/sec. During normal operation hard disks are slow and the CPU show an high 'system time use' during disk access.
There was no difference using bare.i and sata.i kernel. I would exclude the software raid as a reason, because even a test with badblocks on a physical disk gave similar results.
I have the same problem on two different motherboards, both of them with intel chipset: a Tyan S5102 with Intel 875P, and a more recent Asus P4P800-F with i82865PE+ICH5. Regarding Tyan MB, I have not tried promise raid controller yet, but this is not an option, because it would not resolve the problem on the other motherboard.
High system CPU time would suggest a wrong IDE setup (Pio mode instead of UDMA), I tried to adjust something with hdparm, with no effect.

Any idea?

Do you mean what you think you mean, or did you miss type? 30Mb/sec vs 3MB/sec is not apples to apples more like apples to oranges. Bits and Bytes
A bit (lowercase b) is a single character of data (0 zero or 1 one). A byte uppercase B) is eight characters of data. Therefore, eight bits make a byte. So if you didn't misstype your explanation your speeds may not be as different as you think.

[EDIT]
1 megabit = 0.125 megabytes

30 * 0.125 = 3.75MB

It was only a typo (now corrected), both values must be intended in MEGABYTES (MB).
If you do a cat /proc/mdstat in a system with software raid, you can see its status: if one of the volumes is resynching, the system show the resynch speed, using [Mega|Kilo]bytes, not [Mega|Kilo]bits.

Thats fine, I was just trying to clarify for you and others who may not have know is all. Didn't want you to beat your brains in for no reason.