a few minutes ago my system was very slow, because the cpu was very busy, but I could not figure out with what.

'top' had a cpu usage line looked someting like this: (reconstructed)
here the last field, "si" was fluctuating between 40% and 90%, almost nothing in idle or user, making it hard to find what was so complicated to process.

edit: clarify problem.
What do the different fields mean: my guess is 'us'=userspace(or something similar, running programs) 'sy'=system(kernel level), 'id'=idle.
the others I do not know. what are they, is it possible to trace where cpu power allocated to them is going, and maybe reduce them, if too much is going there, bogging down the system.

'si' is software interrupts.
for more details on the fields.

While you're reading the man page...

Have a look at the -b and -n options. You can have top run once and dump its output to a file so you can check the data afterwards and post it here (if necessary).

If you have X running with Gnome or KDE (Gnome is better ), just load up gnome-system-monitor. That'll tell ya pretty much all you need to know...

You'll be guessing, but it's likely the highest CPU consumer is generating all those interrupts.
Sounds like something gone feral.