If you have Compiz turned on, try disabling it when you run the game. You could also try some of these tweaks.
Open Synaptic and make sure to get rid of all "residual config" files.
Clean up "private data" in Firefox, cookies, cache.
Turn off "Services that you do not need, some examples; bluetooth, automated crash reports.
Go into "Sessions" and turn off "Tracker", "Tracker Applet", there may be other you will want to disable but Tracker is known to slow the system down.
Open the Terminal and type, sudo apt-get clean and enter your password.
Here are some additional tweaks that will make things a little nicer for you.
Right click on the mintMenu and choose "Preferences", click on the "Applications" tab, change the "Hover Delay" to 50. (This will make the menu feel much more responsive.}
Change the "Auto-Refresh" in mintUpdate to 1 day. (Every 5 minutes is way too often and tends to slow down Firefox.)
Go into the "Security" tab in Firefox and remove the checks for, Tell me if the site I'm visiting is a suspected attack site, and Tell me if the site I'm visiting is a suspected forgery. (Doing this will help keep Firefox from crashing when using Flash.)
Open Synaptic and install "preload". This will help speed up application launch, keep in mind that preload takes time to learn your habits.
If you do not use Gnome Do, I suggest turning it off in Sessions. (I do not use this feature and leaving it enabled eats up memory I can use for applications.}
Remove any old kernels on your system using Synaptic. Be certain to start by removing the old linux-ubuntu-modules first. (It is extremely important to remove these first.) Take your time and use care and you will not have any problems with this.