If your pc isn't that beefy, then you should try shutting off backround programs, it will cut the lag caused by your system a lot. I know I used to crash every 15 min after I logged on my old computer untill I found out about this.
So check out this post, the first part is about using msconfig and that's what you should try first, then there's a bunch more stuff listed you can try too.
Also, it's hard to run missions solo as a spy at lower levels unless you don't actually have to kill anyone. So best bet would be to do infiltration/courior missions if you're gonna go with spy.
