Game Started being slow since 3 months

I have Minecraft for over 2 years now and I've always played on the same pc for about a year. But about three months ago, Minecraft startedbeing very slow and I deleted quite much all my other games (MMORPGs and disc games). And I installed the latest version of Java. Still, nothing so I put all my graphics down to the lowest quality you can get. But still. Minecraft is functioning very slow.
Everytime I jump, it reacts about 0.8 sec later, same for movement and the mouse. Game works great for the rest but it just doesn't wanna react so fast. I have the same NVIDIA driver now for those 2 years (GeForce 7025 / nForce 630a).. Anybody ideas to solve my problem? Thanks in advance!!