For most applications we recommend a variable step solver, but PLECS Blockset works with both variable step and fixed step Simulink solvers. Note that you can have fixed-rate periodic tasks and a variable step solver. Variable-step solvers still accurately take simulation steps at the discrete control time steps in addition to the adaptive time-steps due to the power electronics system dynamics.
Also, if you are using a variable step solver and the simulation is running slow, evaluate different solvers and check if you should be using a stiff or non-stiff solver.
Simulink also has a "local solver" configuration which you may be able to use, but you should refer to Mathworks support for documentation on that feature. I am not sure if it will be applicable here.