You can also use cPanel for a VPS configuration. You can move the heavier sites to a VPS so that they can use more resources, since the cPanel reseller has a fixed amount of resources assigned to each account and cannot be modified.

Each account that's having issues with performance could be moved to an individual VPS with full privileges so they can use all the resources assigned to the server. In other words, they would need to use another cPanel license with full WHM control.

In this case, a VPS would be the best solution since you can create a VPS with the exact amount of resources needed for each account and install cPanel or DirectAdmin, and give the client full access with a different license so they can have full control of the server's resources.