MacBook Pro 2018 performance issues while connecting to a monitor

My Macbook Pro is the 2018 model equipped with the quad-core 2.3 i5 processor. The performance is usually really good but something really interesting happens when I connect my Macbook to a monitor using a USB-C to HDMI cable.

Problem Number 1:

The processor starts to slow down/lags for no reason (approximately 0,4GHZ!!!) while the temperatures are between 70 and 60 degrees and therefore the mac becomes unusable for a while.

Problem Number 2:

My usual max clock speed without the monitor plugged is around 3,6GHZ (not 3,8 as advertised) and this speed stays stable for the entire time however the instant I plug the monitor in the processor speed becomes really unstable never hitting 3,0GHZ

This issue really annoys me because I really want to use my mac with a monitor but this performance drop really impacts me a lot.