Why does iMovie drastically drop frame rates when speeding up videos?

I'm trying to use iMovie to edit speed paints of my work. I use a 2013 MacBook Air, and it's been updated to High Sierra, with the version 10.13.6. iMovie's current version on this laptop is 10.0.9.

I record my processes then drop them in iMovie to speed them up thousands of times. Normally, I'd speed them up to at least 10,000% faster. However, once I play the sped-up video, iMovie seems to have lowered the quality. It isn't very smooth at all and the video seems to be jump from one frame to another, losing a lot in between.

The first time I did this, my initial video was saved as a .flv file. I had to convert it to be able to edit it in iMovie, so I converted it to .mp4. When I sped the video up with iMovie, I had no issues. The video ran smoothly without looking like it was lagging.

The second time around, the initial video was saved as a .mov file, which meant I no longer had to convert it. I sent it straight to iMovie to speed it up, but this time, it seemed very choppy and laggy. I checked the original video if it was just as choppy, but it turned out to be completely fine. iMovie was the one with an issue.

I speeding it up at different speeds. The default 2x, 4x, and so on were okay, but once they got to speeds such as 1,000% -- let alone 10,000% -- the video just seemed to get choppier every time.

I tried it again with a completely different .mp4 file to see if the format was the issue. The same thing happened, so it wasn't the issue.

After further observation, I noticed that the sped-up video wasn't very choppy at the beginning, but it became more choppy the longer it played. Sometimes, its frame rates differed; it was slightly less choppy on some plays, and slightly more choppy on others.

Could this perhaps not be an issue with iMovie, but rather, that it really is my laptop that has a performance problem?

Thanks in advance. Please let me know if further information is required.