How can I prevent external monitors from slowing my Macbook? Until three months ago, I could use two external monitors with no problem. For 3 months I just used one, but now, trying two, I find that my MacBook Pro (15-inch, 2017, OS 10.15.5) gets slow--for example, my mouse pointer lags. In fact, even with one external monitor there is some lag.
How can I prevent this?
Full configuration is  shown below.
Is the laptop getting enough power?  This model of MacBook Pro has two graphics chips and will shutdown one of them if it sees a need to conserve power.  That could explain the video lag.  Are you using the original power supply?  Is there some device plugged in that could be drawing significant power from the laptop?