Jeez what did you put in it to make it 10 mb? Did you put hundreds of decals? Cuz that'll do it.......
Anyways, it's likely you have entities with physics colliding. If you're, for example, using wood beams as welders or platforms, make sure you check the Set Static Physics box. If that's not the case, then it's caused by your objects falling at the beginning of the map, overloading the physics engine. To counter this, lower your entities closer to their resting positions, and utilize the 'Create on surface' option when placing other entities.
Good luck
Also, if you can't fix your lag, create a fade in sequence on entrance.
void OnEnter() { FadeOut(0); SetPlayerActive(false); FadeGlobalSoundVolume(0, 0); AddTimer("", 3, "FadeIn"); }
void FadeIn(string &in timer) { FadeIn(2); SetPlayerActive(true); FadeGlobalSoundVolume(1, 3); }