it'll be significantly slower. I have an iMac 2008 (2.8 GHZ) with ATI 2600 HD card, Mac Mini and MBP 15 both with C2D and nvidia 9400m. The iMac took the biggest hit, as they don't have decent drivers for Lion for ATI cards. This is a known issue that has been discussed to death on apple boards.
The 9400m machines work OK, but are still slower than they were under SL.
I was at apple store yesterday and just comparing a 21 low end iMac to 27 inch high end iMac - the low end iMac could barely handle the system animations on Lion - they were definitely laggy, not as smooth as under SL. The high end machines could handle things better.
Things are a lot worse on older hardware - my iMac couldn't handle 720p youtube playback, especially after it went to sleep and then I woke it up, due to the problem I mentioned with ATI graphic drivers. Some people were successful installing SL GPU drivers under Lion - not something I'm willing to try.
Needless to say, all my 3 machines are back on SL. I hope they fix these problems with Lion soon, as I'm thinking about buying a new iMac - and right now I don't fell like these iMacs with Lion are faster than my 3 year old iMac with SL.