SUMMARY
I applied the Position and Zoom-effect to a title, and added a keyframe somewhere in the start and in the end. I've set the size to something big at the later keyframe. The transition between the states occurs rapidly, over the time of a few seconds, rather than slowly, linearly over the whole duration of the keyframe-interval. KDEnlive shows the calculated values in the boxes - these seem correct, but the preview-renderer in the App and the rendered video both have totally wrong timing and apply the effect way to fast (I assume, there is a hardcoded value somewhere). 

This also occurs with the rotate (keyframeable) effect. The first keyframe is recognized correctly, but it seems the interval is either calculated wrongly or hardcoded to like 3-4 seconds?

STEPS TO REPRODUCE
1. Create a title (a video clip seems to work fine), stretch it out over like 20 seconds
2. Apply Position and Zoom, create two keyframes, modify the values of the latter; place them some 10s or so apart
3. watch the preview render

OBSERVED RESULT
Effect is applied right after the first keyframe

EXPECTED RESULT
Effect should be evenly applied in the whole interval between the two keyframes

SOFTWARE/OS VERSIONS
Linux-Kernel: 5.4.81-lts
MLT version: 6.24.0
KDE Frameworks Version: 5.77.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION