i have a ps3 slim model (250gb) most games are quite laggy during cutscenes as well as gameplay its not the actual game, because I tested the same game, for example uncharted 2 on my friend's ps3 and it worked perfectly
(another thing, it is not the tv or hdmi cable, as i tested it at my friends house on his tv/hdmi cable) Also another reason i KNOW it is not the game, because most games lag despite being brand new. I tried to go get it fixed as i thought it was a blue ray problem but the person told me it wasn't, as if it were a blue ray problem, the game shouldn't work at all, not be laggy he said it was the first time he saw/heard about this happening, suprisingly with a slim model!
PS: my warranty is over a year now
(this problem was occuring even after jailbreaking) (i have tried a master reset/system restore/or whatever its called)
Even though it would not help with BD discs, try running games off the internal HD and see if they lag. If not, then it's definitely a fault with the BD laser or lens etc. If they DO lag, then something else is going wrong.
If your internal HD is set to SATA 2 then it WILL lag. Depending on the brand of your drive you may be able to find an application that forces your HD to work at SATA 1 settings. Goto the manufacturer's page for your HD and the application will be in the downloads section.