Are there varied bullet speeds according to different guns? Do guns like the AWP have faster bullets than guns like the M249?
CS:GO, along with all other major source engine games, uses hitscanning.
When a bullet is fired it instantly detects where it will hit based on the angle it left the gun (Which is worked out by the guns accuracy, which I wont go into), there's no physics simulation or bullet movement to take into account.
Basically, every gun shoots instantly, there's absolutely no travel time. If you're experiencing something like that, it's possibly lag or a shoddy server.
For clarity: The tracer you see is just a particle effect to make the game more immersive, it has no affect on the shot and can sometimes be misleading as it moves ever so slightly slower than the actual shot (Which as I said before, is instant)