URGENT- h.264 to ProRes issues

A client sent me a series of .mts videos that Compressor wasn't able to recognise. I used an MTS converter a friend recommended, which was only capable of converting to h.264. These videos look fine and I was confident that I could just convert them to ProRes and begin editing.

However, when I looked at the resulting ProRes videos, they look really "laggy" and kind of like they are being played in slow motion. It's really choppy and horrible. I figured it was something like wrong frame rate but everything I've checked seems to match the original video.

I'm absolutely stumped, I've tried pretty much every combination of settings in Compressor but can't work it out.

I can't edit in h.264, but the ProRes files look like garbage.

Please help, this is quite urgent.

Many thanks

Ryan