Update your graphics drivers first of all - AMD - NVIDIA. AMDs beta 13.2 drivers fixes some FPS issues with D3.
Make sure you don't have a lot of other programs running or starting up with Windows. You can disable startup programs by going to Start menu->Search Programs and Files field->Type MSCONFIG->go to the Startup tab->uncheck stuff you don't use->restart Windows.
Then start up D3 again, make sure it's in Fullscreen mode. Set all video settings to the lowest possible, test play some, increase one setting at a time to determine what is causing the lag. Spend some time with it and try different settings. Some things like Shadows for example can have a huge impact on FPS but it really depends on your hardware and drivers. You might be able to have almost everything on full settings except one.. just keep trying different setups.
If all fails, keep googling. Just a quick reply from me, gl.
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"For I know that my Redeemer liveth, and as the last man He shall stand forth upon the Earth."
Me too am experiencing FPS drops since 1.07 hit the servers.
I have a desktop pc with an Nvidia gtx 460 as video card.
All drivers are up to date (video card, mainboard, audio etc).
I got them mostly in act 3, drops from 59 FPS to 10 FPS.
Sewers have some problem since way before 1.07, at some crossing FPS drops from 59 to 30 with no apparent reason.
Unfortunately I cannot advice you about solving this problem, I'm still trying to find some clue.