Lr 8.4 raw 11.4 export issue

Another thing to try: Run Software Update for macOS, and if it tells you that the Supplemental Update for macOS 10.14.6 is available, install that. It may or may not solve this specific problem, but it has solved some other "it's taking way too long" problems in Lightroom Classic.

Changing quality from 85 to 100 on my test export of 75 raws didn't slow it down at all, though the photos increased in size by about 45%. So using 80 or 85 is a good idea to make your photos smaller without sacrificing quality, it may not be causing your observed slowth.

"My setting on export are ... quality 100"

JPEG quality 100 will needlessly make your photos much larger (and slower to export) than a smaller setting that produces the same actual perceived quality, e.g. 80.  See http://regex.info/blog/lightroom-goodies/jpeg-quality

That's about 4.2 sec/photo, which seems really slow. I just timed an export from and to the internal SSD drive on my Macbook Pro and it only took 0.83 sec/photo.

As a first troubleshooting step, try copying those 84 originals to your internal hard drive, importing the copies to LR, and exporting them to the internal drive. If it goes much faster, that suggests there's an issue with the external drive.

How large are the photos (before export) in megapixels? Did you perform a lot of brushing and/or spot healing on these photos?

Ive not noticed any change. I usually export jpegs or use publish services to export to SmugMug. Has this just started happening? What are your export settings?