I was thinking if there is pump that you could add to the hot water
supply to increase the speed of the hot water flow. This is for an
open vented system.

Yes. It is called a single impellor shower pump. If you're pressurising the
whole house with it, get a good quality heavy duty type such as a Stuart
Turner Monsoon. Depending on the layout of the system and myriad other
factors, you may find the increased flow rate leads to air being sucked down
the vent tube. If this happens, and there is no obvious culprit like a half
closed feed valve to the cylinder, you will need to install a flange on the
system (or you might find turning the pump down fixes it). The flange will
normally eliminate the air effect provided it isn't too bad, although it
will effectively reduce the size of your cylinder slightly as the top of the
cylinder will become "dead" space.

Christian.