My answer is that every pc should have malwarebytes on it, it catches more than most any other malware scanner out there. It should not be slowing you down however because it is an on demand scanner and is only using resources when you are actually running the manual scan.
Do you mean it becomes slow when you are running a scan or it is always slow? You might try doing an uninstall of it then a re-install. but since it only comes on and uses resources when you actually run a scan with it I can not see why it would have any impact at all on the pc.
It could be just coincidental that it slowed at that time could be something else contributing, I would look at the task manager and see what seems to be using the most CPU resources.

You do not need Malwarebytes but only you can decide the level of protection you wish to have. It is a very good back up program if you need to feel secure and can then be ran at your convenience. MSE scans every Sunday unless you alter the settings and updates as the updates are created, all fully automatic.

I like Malwarebytes and have used it on all my computers except this one which only had MSE on it.

" I would look at the task manager and see what seems to be using the most CPU resources.".. Ravencajun

Do I check the processes, services, or Performance in the Task Manager??? Sorry I don't know how to check for resources. Advise please....

Depending on whether or not you have the paid version .. Malwarebytes does run in the background.

I have the paid version and sometimes it Will slow some pages down when loading.

I think that it has a learning curve.

Ever since I uninstalled FREE Malwarebytes' AntiMalware on my Laptop..it is back to normal and running fast. I Disk Clean, Scan, Frag, etc. my computers faithfully. I like Microsoft Security Essentials...and went on their site and asked them the same question I asked here......They said you do not need any other malware with MSE as long as you keep your IE, Windows and MSE updated and have Windows Firewall. Plus never click through links from unknown sources..Practice safe web browsing -- which I do.