Media Encoder CC 2018 Very Slow

I'm having the same issue since upgrading to Media Encoder CC 2018 a few days ago. Previously my 10 to 20 second videos were rendering in no time. Now the exact same videos are taking ages to render or not rendering at all.

For me the main issue is that they take 5 minutes to begin rendering, and I frequently get 'Cannot read from source' errors, or Media Encoder just hangs. Prior to updating the software I had no issues.

Software only or GPU Acceleration makes no difference.

MacBook Pro (Retina, 15-inch, Mid 2014)
Processor: 2.5 GHz Intel Core i7

Memory: 16 GB 1600 MHz DDR3

Graphics: NVIDIA GeForce GT 750M 2048 MB

Intel Iris Pro 1536 MB

Adobe Media Encoder CC 2018
Comps created in After Effects CC 2018, which I updated at the same time.

Any advice would be greatly appreciated.