Well first post what your PC specs are...
Second, I'm gonna assume you play zombie escape or ttt. When you're on zombie escape, the server does reach insane laggy moments, IE when we're on ze_santaassasination, with all the particles and shooting and powerup going off at the same time, the server does lag quite a bit (net_graph 1 shows the sv going wack), however an underpowered PC would also not be able to handle everything happening at once.
TLDR: It can be both server and pc lag.