After installing malwarebytes my laptop's performance slowed down seriously. What's up?

Rare but can happen. Since there is little known about the laptop I can't make a guess. I will share this mostly seems to happen on older laptops where the boot drive is old technology HDD. Tell more about this machine. Also, uninstall MWB to see if it resumes performance. I also will guess it's on Windows 10 so you already have some protection in Windows Defender so there's not much reason to enable the real time protection of MWB.

I would also be interested to know if there are other (Norton, McCaffee, etc...) anti-virus / anti-malware programs installed. It has been proven that two (or more. LOL) such programs reduce efficiency of the said programs and greatly increase use of system resource. To overly simplify it: they see each other as potential malware. As rproffitt said: please post the make and model of your laptop, any upgrades you have made ( more RQAM, SSD, etc...) and a list of installed programs.

We have stopped using Malwarebytes for precisely this reason. It doesnt seem to work well with other anti-virus/firewall software on a day to day basis. We now only use for an absolute emergency.

Just to note that this is even on up to date laptops with SSD boot as well.

Not very helpful, I am afraid, but you get impatient after a while, especially when everything else seems to work quite happily wihout it.

Interested to know if anyone actually finds a way it could be brought back into more general use.

Best wishes

Richard