Hi Markus,
Sorry for the delay.
I have tested the following page using a virtual machine with IE8 and it seems to work fine:
http://www.daypilot.org/sandbox/Scheduler/RowHeaderColumns.aspx
I haven't noticed any performance problems (but this page doesn't display many events).
Would you be able to create a simple test project that would reproduce the problem?
I know this isn't what you want to hear but IE8 will be dead in a few months as Windows XP is going to be phased out. And you can't expect the greatest performance from a 5-years old browser. Anyway, if this is something that can be fixed, I'll try to do my best.