I have been wondering this for quite a while, thought I might give a shot asking it here.
I use an application that used a lot of resources (for example image batch processing or compiling/running a lot of apps)
Close the resource-hungry apps
Ubuntu still significantly slower than before the resource-hungry apps were started*
* I mean, 'top' indicates low cpu usage, until I do something (as trivial as opening a tab or something) and my cpu usage spikes instantly. I noticed this on a multiple of Ubuntu boxes I own.
A reboot is the only solution I found. What's the reason behind this?
EDIT: a bit more info about my laptop: Dell Studio 1558, Intel i5, 8GB DDR3 Ram, 7200 rpm HDD