Darth Plagueis is underrated , he obviously was powerful or Sidious would not have killed him by getting him drunk and killing him while he slept . The Fact he remained control over Sidious for so long and well into his old age says something as well . We can take the wait as a want to maybe get all Sidious could out of him in Sith knowledge but he waited a really really longtime to take the Title Master .
I believe Sidious never really wanted to be under Plagueis' thumb anyway. It is the nature of the Sith to kill the master and take the title from him. Plagueis had come to trust him, in a way, eventually giving him more and more knowledge of the Sith. Eventually, Sidious saw no more use for Plagueis and decided to kill him. It was about time too, as Plagueis was going to keep Sidious under his thumb for the rest of his life.
Just to point that tidbit out.