Not a lag problem. In most circumstances, sorc and warlock tether heals can outheal a single bolter, though a bit less so with warlocks (squishy eldar). Lash of Slaanesh has technically unlimited ammo, but functionally you can get off 5-6 shots depending on your build before you have to recharge your warp energy. It's range is also limited to about 25 meters, and on top of that it has a short wind up and is not perfectly accurate. It takes some real skill to use, but in the right hands it is deadly. Keep your distance from Slaanesh sorcs.