braw files

Hello, I am working on a 2017 Mac Book Pro 15in 2.9 GHz, 16GB 2133 MHz.

I am working with Black Raw Magic files in Premiere Pro. After exporting as a Match Source H.264 the playback on QuickTime is lagging, this is also the case on my phone, but when I send it to people it runs smoothly. I have plenty of storage left on my computer and iCloud. Not sure if this is my Mac and phone or what. Any help is appreciated. Thank you.