A while back, I checked out what I could find on the internet with regard to original TMP reviews from 1979. Positive ones, like Roger Ebert's, tended to compare it favorably to 2001, etc. The negative ones usually felt it was over-reliant on special effects or that the plot seemed lifted from old Trek TV episodes, but I don't remember a lot complaining about the pacing - the closest I can remember was one reviewer thinking there was a little too much of watching the crew react to stuff on viewscreens for his liking. Probably because movies (and TV) in general tended to be slower paced forty years ago.
But I'm with J.T.B. that there did seem to be a big backlash towards TMP in the 80s. I'm sort of curious of the demographics of such: was it largely from folks who were introduced to Trek through reruns as kids in the 70s, weaned on Star Wars, and "came of age" (if you will) as adolescents in the era of whiz-bang 80s blockbusters, which made TMP seem like even more of a dinosaur?
It's also possible that TMP suffered retroactively by comparison to WRATH OF KHAN. When TMP debuted, there were no other Trek films to compare it to, so sympathetic reviewers compared it to 2001.
(I also believe that there's a fair amount of CLOSE ENCOUNTERS OF THIRD KIND in the movie's DNA, but that tends to be overlooked as people forget what a big hit CLOSE ENCOUNTERS was at the time.)
But when KHAN came out, the conventional wisdom quickly became that it was the better STAR TREK movie, so people started lauding KHAN at TMP's expense.
KHAN was the success, the movie that took the right approach, whereas TMP was a noble failure . .. .