Go to File > Project Settings > General and check to see if you still have Mercury Playback Engine GPU Acceleration Open CL enabled which should be on by default for your system. You could try switching to Metal or Software only to see if there’s any difference in rendering time. Any switching might delete the previews you may have already rendered.
You could also press queue from the export window and have Media Encoder do the export for you. It also has an option on the bottom to switch between the renderer as well.
Have you been using the same external drive for all of these exports? It could be wearing down. I’ve even heard of some new drives that start to fail. Very rare but it happens. I’d try bringing all the files to a new drive to see if there’s any difference.
You could also use certain programs like the black magic disk speed test to determine how fast you can read and write on your drives. Check all your drives, external and internal.