You're going to have a hard time editing any 240fps video, simply because there are so many frames that have to be decoded every second. It's one thing to simply play that back, it's a completely different animal to edit.
You can try using shadow files, or going with non-real time preview (which will play slower than full speed, and will have no audio), or only previewing a few seconds of video at a time, then stop and let PD "catch up."
If I were you, I'd use VirtualDub2 and convert to MagicYUV which will give you full resolution, full frame rate clips to work with directly. The only downside is that you'll need tons of room on your SSD because MagicYUV clips are uncompressed and will need 10x-20x more space than the source clip.
Your produced clip will be
much smaller, because you'll be using H.264 or H.265 compression, so it's only the intermediate stage where you'll need all the drive space.
YouTube/optodata
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