Before blaming the plane, would suggest looking at the roll first.

Any roll takes a short amount of time to initiate, and if same controls are held in place, should speed up slightly, wind conditions ect not being a factor. So what you are saying sounds reasonable.

Secondly, when trying to do a roll with precision, it is different. If you are adding elevator at appropriate spots for a linear roll, it is very very common for people to add more aileron with the stick as the roll progresses. This is responsible for changing the roll rate.

Just some thoughts that may be occurring. With the plane on the ground and everything on, try doing a roll in your mind and see if the ailerons are affected when you bump the up and down elevator. Work towards moving the sticks without affecting the other surface. Add ail and see if the elevator moves and vica versa, move the elevators and see if the ail move also.

Ed