I'm not fast or a strong sprinter but I routinely beat cars into the intersection from stops, and often have to brake behind the lead cars when I take the lane for left turns. Unless the driver is particularly aggressive and floors the accelerator I have no difficulty matching or beating them into and midway across a typical intersection.

But maintaining that speed is another thing. Usually by the time I've reached the other side of the intersection most drivers are passing me.

The point is, I don't find it unsafe to take the lane for a left turn, either from the front of the lane if I happened to get there first, or behind a lead car. As I noted before, in busy traffic and complex intersections I prefer to get behind the first or second car in the left turn lane and let them break the path. As I complete the turn safely the flow of traffic is beginning to pick up speed and I'll merge toward the right to minimize delaying cars behind me. But often I need to brake or pedal less aggressively, because the cars in front aren't going as fast as I'd prefer.