Background rendering slow, foreground rendering fast. why?

So, i’m encoding a lot of stuff usually while working on other things. but i can’t. Here’s why:

As soon as i’m switching to another application like writing a mail, or going to finder, my processor clock goes from full boost clock (5ghz) to 1ghz. As soon as i change to media encoder it goes back up and it renders like 5 times faster. 

So as i know, media encoder is "Made for transcoding in the background" but since it is not performant in the background it makes no sense. 

Any workaround here? is this a feature of MacOs or is media encoder to blame for this? 

Best Regards.