I recently got a 4K external monitor (LG 27UK650) for my MacBook Pro (Retina, 15-inch, Mid 2015). I mirror the display to the monitor and use the keyboard and trackpad of the Mac as the Magic Trackpad 2 seems super expensive and I haven’t found a keyboard I like yet (my first choice, Logitech K811, has been discontinued and my second choice, Logitech K380, is sold out.)
I starter noticing extremely sluggish behavior, increased fan speeds and high CPU usage when connected to the external monitor, which it seemed like it could be because of blocked air vents and dust accumulation. I opened up the Mac and cleaned it out with a can of compressed air. The Mac runs much quieter now but the issue crops up, albeit after a longer time, once I connect the external monitor. I now plan on replacing the thermal paste as I live in a hot climate and it looks like this could help. I have ordered the ARCTIC MX-4, which seems to have overtaken the Artic Silver 5. This is the first time I am going to be attempting something like this and so would love to get tips and tricks from the community!