Something to remember is that it can be highly variable between devices and people.
I installed 10 on my 920 multiple times (bounced between 8.1 and 10) and sometimes the upgrade's performance was terrible before a hard reset while another upgrade worked fine.
I just threw it on my 830, which has the same internals as the 640, and it seems to be perfectly usable (I'm often picky about responsiveness so its a good sign) save for some very minor twitchiness.