[SOLVED] Kubuntu seems to need for than 4 GB of RAM

Is it just me or does it seem like Kubuntu needs more than 4 GB of RAM?
I have Kubuntu 18.04.4 on multiple PCs.
One has 16 GB. The other has 24 GB. The other has 4 GB and another has 4 GB.
After using it for a while, I thought QT Creator was to blame. There would be lots of HDD light activity, the mouse doesn't move or barely moves, the time sometimes does not update.
This could last from 30 min to 5 h.
The PC is unusable and I have to wait or just pull the plug.

However, when just using Firefox for youtube viewing and a few web page searches + using Occular to search in a PDF file yesterday, it happened again.
Libre Office was also open.
I think I had Steam opened as well.
It became unusable for 1 h.
Lots of HDD light activity.

It looks like the HDD has a very high priority in Kubuntu. It disregards the GUI and mouse.

I am trying Linux Mint. So far, so good.
10 Firefox tabs open. Terminal open. Libre Office. xed for text editing. Xreader for PDF viewing.

I have to think the average software developer stopped using a system with as little as 4GB RAM years ago, so running multiple RAM hungry apps at the same time with a 4GB constraint just doesn't get much developer testing, unless by those producing distros specifically made to fit low RAM and/or older hardware, e.g. AntiX.

On openSUSE 15.1 running KDE3, I just closed Firefox. Disk cache dropped by 1%. App data dropped by 23%, so total RAM dropped by 24%. I have 16GB RAM, so that's just a little less than 4GB of RAM freed. Closing Chromium, which had fewer open tabs, freed 7%, around 1.1GB. I have to think FF + Occular + LO + Steam are likely to fit poorly in 4GB, so are likely to need to utilize swap.

Mint might be more efficient that Kubuntu's KDE5 Plasma. Mint uses a completely different DE than Kubuntu, while Mint is built on Ubuntu, which in turn is built on Debian. In essence, changing to Mint essentially amounts to a change from Plasma to Cinnamon or Mate or XFCE.

On Debian or Ubuntu I use TDE, which is a KDE3 fork that I find very efficient with as little as 2GB RAM.

I see.
I guess I'll leave it with Linux Mint on that machine with the 4 GB and later on, install Linux Mint on the other one with 4 GB if things go well.
I already tried to update the RAM to 2x 4 GB = 8 GB and also tried 2x 8 GB = 16 GB.
The BIOS does not recognize it. The BIOS is already up to date. It is a limited HP machine with a Core2 Quad.
The CPU is strong enough for compiling code, watching movies and such so I don't want to get rid of it.

Consider this thread SOLVED