While searching the Internet for an solution for one problem I had, I accidentally found something, that can be useful for everyone who use IE8 as his primary browser. Manual registering one dll library can improve the IE8 rendering performance. The reason for that is unknown, because the dll library had been registered during installation of the browser (without it the IE8 cannot run). The magic command to execute is:
regsvr32 %windir%\system32\actxprxy.dll
After executing it log off and then log on again – the IE8 performance from now will be visibly higher.