Serious Lag when exporting via ME

Hey everyone, 

Recently I have starting working from home so I have being using my personal iMac for work purposes (video editing) so I updated all my software & Apps on my iMac to make sure it's up to date for any video editing projects that might be passed my way. 

I have recently being editing two videos for a client, both around 12mins in length and both with multiple layers within PremPro. The issue I am having is that when I come to export the videos and send them to the ME que, as soon as I start the exports my entire iMac slows right down to the point where I can't even use it until the exporting is complete. I'm only exporting at HD 1080p so nothing too crazy. 

iMac specs:

iMac (Retina 5k, 27-inch, 2017) 

Processor: 3.4GHz Quad-Core Intel Core i5

Memory: 16GB 2400 MHz DDR4 

Graphics: Radeon Pro 570 4 GB 

I have all my cache and scratch files set up to a separate 2TB usb 3.0 external hard drive. 

I have never had this issue with this iMac before until recently when I have updated all my Adobe Apps. It usually performs really well and fast. The even stranger thing is, Iv recently being working on a graphics project for another client also within AE and when I render out that project within AE my iMac runs absolutely fine. Even when Iv exported a .mov file to a .mp4 in ME it's not slowed down one bit. 

Can anyone help or suggest anything to try to help with this issue? Or is anyone having a similar issue?

I have attached a short video of the iMac screensaver during an export from ME and as you can see it is very jolted, usually that screen saver has a very smooth fluid motion. 

Thanks in-advance,

PhilSparq