I seem to have fixed the issue that I was having with AE being really sluggish now!
I ran a www.userbenchmark.com test to see how my PC was performing and it came back with this disappointing result: https://www.userbenchmark.com/UserRun/26005368
I noticed that the processor wasn't running anywhere near as fast as it should do, I checked my power settings in Windows Settings > Power and Sleep > Additional Power Settings and saw that my PC was running in power saver mode. I changed it to High-Performance mode instead and suddenly After Effects came back to life and the lag is now gone.
The results are clear in the userbenchmark.com test that I did after this 😄 https://www.userbenchmark.com/UserRun/26005636