I have a Lenovo y570 with a Nvidia Geforce GT555m. I just recently bought GTA IV on sale from Steam, and I'm having the same issues with it that I was having with Crysis. I originally thought my system wasn't beefy enough to run Crysis but after having the same issue with GTA IV, I've come to believe its caused by Optimus.
When I play either game, I have initially start out with high FPS but in Crysis, it drops after a few minutes to under 20. With GTA IV it happens instantaneous. When I selected Benchmark on the GTA IV graphics menu, I noticed after the benchmark that the game was only recognizing the Intel HD graphics. I assume this is the issue with Crysis as well. It seems to only happen with older games, HL2 included. Doom 3 runs off of the GT555m no issues.
I have tried updating to the newest available drivers and selecting the games individually from Nvidia control panel but neither helped.
I found a fix! The CPU was throttling horribly so I downloaded a program called throttlestop and set it to the "game" preset. Now I'm playing GTA IV at 25-40fps and for the first time Crysis runs on my laptop without dropped below 20fps!!! So exicted, I've been trying to play Crysis for ages now.
It didn't overclock the CPU, I have the GPU overclocked with EVGA Precision but the Throttlestop program just gets the CPU to run at a solid pace. I went from 10-15fps to 30-40fps even though GTA IV still shows the reads the intel card during the benchmark.
Crysis is running much better aswell, went from 15-20fps and huge lag spikes to smooth running at 30-35 fps, Crysis Warhead runs at close to 50fps. Both games were unplayable before I downloaded throttlestop.
Don't know what to tell ya. From what I understand GTA IV is a poorly optimized console port. It doesn't recogize the Nvidia card so it isn't compatible with optimus. I have an Intel HD 3000 on my laptop and I can get it run on med-high at 30fps with the tweaks I have told you about. I have gone as far as to try and disable the integrated graphics which crashed the screen to black until I rebooted. I guess that means nvidia optimus works by combining the Intel HD chip with the Nvidia chip. When graphic intensive programs are detected, Optimus switches to the dedicated chip and when you are browsing the web, etc the intel chip is active. Since GTA is a console port it probably doesn't even detect anything other than the Intel chip, and from other forums I've read Rockstar support isn't much help.