Now that you mention it some of my cameras may be affected by the same issue. For example I have a 4MP PTZ running at 30 FPS, iframe interval 30 for both main and sub streams. Its clips begin with a smooth 30 fps for a second or two, then dip down as low as 10-15 FPS and suffer from a very inconsistent frame rate for the remainder of the clip. The same behavior is witnessed when playing the same clip in the local console. Exporting the clip without re-encoding, the clip is fine and plays smoothly, indicating that the problem is occurring when
Blue Iris tries to play it.
I suspect that the use of sub streams is partially responsible for this, because Blue Iris tries to do something silly like use the timecodes from the sub stream in order to play the main stream, which is simply a ridiculous thing to do but nonetheless I think Blue Iris does it (hence their recommendation to match the FPS and i-frame rates on main and sub streams). However even on some 2MP @ 30 FPS cams I do not use sub streams with, I get significant frame rate drops during playback in UI3 (down to about 23 FPS and back up to like 29 FPS it seems). Not as bad as my 4MP PTZ that drops to 15 FPS or below.
I don't know of any solution for this. Frankly Blue Iris has never been optimized for smooth playback. In my experience, frames are dropped regularly and often during both live and recorded playback which causes playback to be very choppy. I have always just ignored it because smooth playback is not a requirement for a video surveillance system. It would absolutely be nice to have but I can't remember a time when Blue Iris has been as good at smooth playback as
literally any other video player.