I think Splitlip probably hit it on the head...
I also have both Atomic and Apollo Bios. Going from paddles to splits, you don't have the sense of "torque" that you have with paddles, and as a result try to overcompensate with the force of the kick.
Let the fins do the work... they will... An easy soft flutter will get you going faster, and more consistently than you think... You'll get used to that seeming low end thrust not being there, however, in time, you'll find you're moving faster and with less effort...
Just a matter of "unlearning" what you're used to... and learning a different approach...
The entire reason I switched to splits was cramping... They simply went away with the splits...