My macbook Pro 16(2019) SLOWS DOWN when I add a second monitor

This computer was built with a ninth generation 14nm Intel processor. The only way to meet the target performance was to make it a six or eight core processor. This processor generates an enormous burst of heat when it does Turbo Boost, and there is only one cooling rail shared by both the CPU and GPU. So getting Either side too hot ramps up the fans.

This processor was supposed to be an eleventh-generation 7nm processor, but intel is three years late, and is only starting to ship its tenth generation 10nm processors now.

Apple DID redesign the cooling system for this specific Mac. New, high-efficiency fans are used. But for some uses, that is still not enough.

Some of the best advice is to install Turbo Boost Switcher and turn off CPU Turbo Boost. This reduces the huge sudden overheating brought on by Turbo Boost, that simply does not translate into real-world performance gains.

For external displays, changing away from HDMI to DisplayPort family (or to DisplayPort over ThunderBolt or USB-C) will reduce the Voltages and slightly reduce the heat generated. It is a good thing to do, but it may not be enough to solve this issue by itself.

How to fix your 16-in MacBook Pro (Don’t return it!)

https://www.youtube.com/watch?v=EkmdHVfk4XE

Hi StephenHalker,

Thanks for using Apple Support Communities. It's understood that you experience issues with your MacBook Pro when it's connected to your external monitor. Does your Mac ever run slowly when it's not connected to the monitor? Does this only happen when using these two apps? If so, take a look at the following article and let us know if this helps.

If your Mac runs slowly

Kind regards.

StephenHalker,

Thanks for confirming this bit of info. Since it only occurs when it's connected to your Mac and you've already done some great troubleshooting, we'd recommend connecting with the manufacturer for further assistance regarding this matter.

Cheers.