The frame count while rendering previews is inaccurate
This has been an issue for as long as I've used Premiere, so several years at least.
When rendering previews, Premiere describes the progress as "rendering frame _ of _". This progress is completely inaccurate. It appears what premiere is doing is assuming every preview it's rendering is the same number of frames. So if I have 100 previews, and it completes 99 of them, the frame number reported here will ALWAYS be 99% of the total number of frames.
This is obviously a major problem when the previews have significantly different lengths. For example, I have a long movie, where most of the previews are short VFX, maybe 100 frames on average. Then I have an extended sequence at the end of the movie that is several minutes long that requires rendering. When I tell Premiere to render all the red bar sections for the entire movie, while estimating frame count and time remaining, it is completely inaccurate, zooming through the progress and then appearing to slow to a halt when it gets to the final preview. This wouldn't be an issue if the frame count was reported accurately, which would change the % done, and the estimated time remaining to be more useful measures.
Like I said, this has been an issue for several years now and really should have been fixed long ago.