I agree with Lethrface. Maybe if you were to compress your data and ask other people to check out if the game lags, you may have already figured out that it most likely is your computer.
Some things that may help prevent or cure lag: (most likely to less likely)
- Check for virusses and spyware...
- Clean your hard-drive and/or do a defragmentation to regain some space.
- See if your graphics driver is up to date, otherwise update... (may want to check out sound drivers too)
- Do a video-test to see if your graphics-card is still working fine, otherwise replace.
- Do a mem-test to see if your memory is still working fine, otherwise replace.
- Do a speedcheck to see if your processor is still working fine, otherwise replace. Also, check if it doesn't get too hot as that may slow down processes a lot. If it gets really hot, replace the cooling with something more powerfull.
For hardware, you might want to consider replacing it anyway if you have a really old computer.
If none of the above resolved the lag, you should see if your power core is still working fine. If your computer doesn't get more than enough power, performance may go down a lot...
If this all doesn't solve your problem, you should try the tutorials to see which version of RM lags the least.