Judging by the graph, the controller looks insufficient to fully power the motor. Likely it could use at least 100AMP controller, which would form a graph similar to my posted one.

15A vs 60A controller
Underpowered controller looks similar to your graph.


"Underpowered " controller provides a much more efficient-cooler running motor system.
Surprisingly, the lower powered controller provides the same top end power and speed.

I've run one of my 24V eZips at 36V for 4000+ miles, with no overheat problems.
I credit the motors longevity to replacing the 35A 24V controller with a 25A 36V controller. Underpowering the motor at the inefficient lower speeds. This lowers amps, raises efficiency and greatly reduces damaging heat production.