It’s just a windows recommendation, but it depends on the screen size & screen resolution. If 125% is too low, then go 150% or even 200%.
As for the slowdown when connected to an external monitor, that could be due to a number of factors. One possibility is that the additional display is putting a strain on your laptop's graphics card or processor. You may want to check your computer's specifications and see if it's capable of handling multiple displays.
Another possibility is that the display settings are not properly configured for your external monitor, which could be causing performance issues. You may want to check the display settings on your laptop and make sure that the resolution and scaling options are properly configured for your external monitor.