I am using KM 1.6.0 beta2 on Win 7 (32 bit) and I don't think I see any slowdown using accelerator keys. These keys are not something that I use often, so I will have to test to check.
changing tabs (ctrl+tab), opening the rebarmenu for websearch (alt+s) and typing in the address bar (alt+d),
I tried (alt+d) and that highlighted the URL in the address bar instantly.
(alt+s) brought up the 'Sessions' menu instantly.
(ctrl+tab) took me to the next tab instantly.
Typing a search word in the URL bar and using (alt+s) still brings up the 'Sessions' menu.
What operating system are you using? Do any other applications on your machine seem to be slow using accelerator keys?