Try closing all other programs open... and if you have iTunes, go to task manager and kill the processes.
I kill like 10 processes before playing Lunia on my slow computer; it makes it so much faster since you don't have to load all that extra junk.
And OMG kill REACTOR when you start playing... that piece of crap gives so much lag.
If you did all that... then just turn settings lower.
Make sure your anti-virus systems aren't scanning either... and that your computer isn't downloading any updates... raeps it worse than running 10 other programs D: