FCPX export stuck

Hi there,

I'm completely new to FCPX. I mainly used it for one reason now: I had an 1080p MP4 file (about 2.5 hours long) in which video and audio didn't sync. So I took it to FCPX, detached audio, dragged it to the right position and then I wanted to export it. Obviously, if exporting with FCPX only, the file turns out at 100+ GB, so I'm using Compressor 4 for Apple Devices 1080p (for my ATV3). I've tried using both "Export using Compressor settings" and "Send to Compressor", but it always gets stuck right at the start. I can't tell for sure at what % it got stuck at my previous trials, but right now it's at 9%. It went there pretty fast, in under 1 minute, and from there the remaining time just gets longer and longer. It took me over 15 minutes to get from 9% to 10%, which is pretty weird considering that my MBP isn't very old and CPU and memory aren't used over 50%. The only thing that's really changing pretty much at the beginning is that it starts off without audio for about 5 seconds, and then audio comes in.

Any ideas what could be going wrong or what else I could do?

I'm not running any other programs, except Safari right now to type this. I got an MBP13, mid-2012, 2.5 GHz i5 and 8GB RAM - I think that should work out, right?

Thanks in advance for any suggestions!