First thing I need to know what do you mean by "lag."
"Lag" (not really) in this context could mean:
-Delay between moving the mouse and the actual guy turning on the screen (can be related to vsync and fps issues, since it takes a frame from receiving mouse input into the computer, then another for input into the program, then another for rendering effectively making it 3 frames delay, which annoys me most at low fps). This is also related to shooting, and the delay for the muzzle flare to appear along with its sound.
A potential fix for this is your CPU usage. Check your task manager or related to your OS, for tremulous' cpu priority. If its higher than 'normal' then that could be the cause (renders the game over keyboard input)
-Lag - The real thing, related to ping (shoot, there's a few msecs before a bullet hole or projectile to appear), fixed by joining a lower ping server
And just as the others said:
-Sensitivity (Move a lot, turns almost nowhere, or maybe the opposite, too high, small movements makes the intervals which you aim clearly visable and makes long range shooting annoying), can be fixed using menus or the cvar "sensitivity", "m_yaw", "m_pitch" although the last two - you don't want to touch.
-Mouse Acceleration (cl_mouseaccel - If above 0, the faster you move the mouse, the faster your guy turns than it should. Recommended '0', and if it's below '0' you're gonna get screwed when turning fast.
Macintosh users should put "raw input" [forgot cvar] or set their macintosh [system, not game] sensitivity to lowest to prevent mouse acceleration from effecting game. There's also a third-party program capable of hacking the mouse so the mouse doesn't have acceleration at any sensitivity)