Maybe your difficulty lies in both areas: tempo and articulation of the fugue. How much experience do you have with Bach polyphonic works? How many of the Two- and Three-part Inventions have you played, for example?
What I've played from Bach...
WTC 1 Prelude #1 in C major
WTC 1 Prelude #2 in C minor
WTC 1 Prelude #5 in D major (https://www.youtube.com/watch?v=XsVFQ_suk7k
WTC 1 Prelude #6 in D minor
WTC 1 Prelude #15 in G major
WTC 1 Prelude #16 in G minor
WTC 1 Prelude #21 in Bb major
WTC 1 Fugue #2 in C minor (https://www.youtube.com/watch?v=5EJsSUNqGfw
WTC 1 Fugue #5 in D major
Invention #13 in A minor BWV 784
Toccata and Fugue in D minor BWV 565
As you can see, I only have experience with three Fugues. The most difficult Bach piece I've played thus far was the WTC 1 Prelude #5. But I find this Fugue #16 more difficult.
In the RCM syllabus, the Prelude and Fugue in C minor (Bk I), is level 9; 11 Preludes and Fugues from Bk I are level 10, and the remainder are diploma level. The G minor is in level 10, making it among the easier, but not the easiest pair.
I find this very interesting, because it groups them together. I know - they definitely belong together - but sometimes the difficulty between a Prelude and its Fugue are extreme. A perfect example of that would be #5 in D major. The Fugue is IMO one of the easiest in the book, and the Prelude one of the most difficult ones.
What, in your estimation, makes this fugue underrated in terms of difficulty?
It's not that the Fugue is terribly
difficult, I've only worked on it for a couple of days, but it takes time to feel smoothly under my fingers. More time than I expected. I needed significantly more time than usual deciphering the sheet music to figure out which hand plays what (like bar #7), it was funny at times. And finding the right fingering is always troublesome for me when it comes to Bach. I even had fingering problems with the Prelude (#16).
When I say underrated regarding the Fugue, I guess it's one of those pieces - at least to me - which sounds much easier than it actually is. At this point I can play it fairly well until bar #14. The rest I didn't work on that intensely yet. I can see that bars #14 - #18 will cause me the most trouble in the entire piece.