Media Encoder unusually slow

Hi all!

I've spent most of yesterday and last night creating proxy files from my camera footage and it has worked fine but there is one clip that is taking absurdly long. It's from the same camera, same settings, etc but while other clips of similar length have taken 30-45 minutes to convert this one is going strong at 9 hours and planning to need another 12! This seems absurd. Any thoughts on what might be going on?

The footage is shot on a Canon R5, mostly at 50 fps (some 25fps), 3840x2160 interlaced. I'm converting from MXF OP-1a AVC to just the standard ProRes QuickTime Proxy-Half that Premiere defaults to. 

omg I just noticed that it has misinterpreted all of my footage as progressive?! So it thinks all my raw footage is progressive and is therefore matching the proxies to also be progressive. Is this a whole other nightmare or possibly related?

Anyhow, I'm on a MacBook Pro 2019, running Sequoia 15.3.2, 32GB Ram, 8-Core intel i9, all the newest app updates installed. (I don't know what is useful information here sorry)

thanks for any thoughts!

Cat