Well, I tested bitdefender for some time in the past, and I noticed that his protection against PUPs and adware is not one of the best. Bitdefender + malwarebytes means a really Good protection against malware provided by BD signatures, active threat control and anti-ransomware and a Good one against adware. This is one of My favorite security combos which I would recommend, although I don't know which would be the impact on the system in terms of slowdown. It's up to you if using the Premium or Free version, but I have a little recommendation for you: let Bitdefender HIPS on permissive (deault settings), because if you turn them up on normal or aggressive, they will conflict with malwarebytes (If you are installing it with HIPS on normal or aggressive mode, the installation process will be killed. Someone also wasn't able to update malwarebytes signatures with HIPS on normal or aggressive) and also cause problems to lots of other programs