For example, I have some 4k60 videos in my timeline. I use the Video Speed Designer to set them to have a 0.500 speed multiplier. When I play back the "elongated" videos in the preview, it looks nice and smooth at 30 fps. However, those clips look to be dropped down to 15 fps in the rendered file, even though the output is set to 30 fps. The same issue arises when outputting at 60 fps.
Is this a known issue? One workaround is to disable the 'Fast video rendering technology'/'Intel Quick Sync Video' option, but that presumably slows things down. I'm ok with that if need be, but I don't want to be sacrificing rendering speed unnecessarily.