I use iPhone 14 video (vertical), and choose Video–>Vertical in the app. The exported video starts with normal speed, but after few seconds, it slows down significantly. There is no such issue if I use the MS Clipchamp.
It does tell me to convert to another format when I import the file to the app, though I usually ignore it.
Look at Export > Video > Frames/sec. That is your problem; it is way too high. You can change it here in Export (in this version, click the Resample button and No in the dialog to enable it). I recommend to use 30 since your Properties screenshot shows that.
In the future, you can use our Settings > Video Mode > Non-broadcast > Vertical HD 30 or 60 fps. Or, if you prefer your custom video mode you can fix it. If you saved the custom video simply choose Add… again and use the same name to overwrite it.
I should know this number. I am a novice video editor and I take defaults most of the time
It did solve the problem, just tested.
" You can change it here in Export (in this version, click the Resample button and No in the dialog to enable it)"
FYI, it did not allow me to change to 30 or 60, it stays at 80. But the second method works.
I should have asked this earlier and I will spend more time reading all the good document here.
Oh, right. Due to how this works (resample frame rate) and some technical limitations therein, there is a limit to the amount you can reduce it—not more than 1/3. Glad you figured it out.