Dropped frame rates in XAVC S mode.

After importing video shot on the Sony 4k action cam in XAVC S mode that it supports, I'm unable to play it back clearly in FCPX. It says frame rates dropped. This also happens in 1080p at 120fps (because it's shooting at a high bit rate, also in XAVC S mode). I'm wondering if there is something I can do so that I can edit these videos without having to go back to MP4 shooting, or if it's the computer that's not powerful enough to handle it. I have a macbook pro from 2009 that has been upgraded to 8 GB memory, and a 512GB SSD. I cannot upgrade the RAM anything further, but I'm wondering if I can solve this, or if I'm out of luck.

thanks,