the problem is from some items that gliches, most common from M4A1, some parts of it, like handguard start flickering (textures move) you can see it if you press tab. If someone is close with that weapon, your fps will go to 1-2 fps or even game freeze. Same happens if you have the weapon and someone comes close to you (even proximity, like 100m away).
Having a squad of 10 ppl and tested this.

Temportaly fix, till you relog, is to pain the flickering parts or change the parts that are bugged, handguard and buttstock.
buttstock bugged: http://dayz.gamepedia.com/M4_Buttstock_MP
handguard bugged: http://dayz.gamepedia.com/M4_Handguard_Plastic