I just got a new iMac and have installed PremierePro 2021. Upon importing my first video clip (shot on a Nikon DSLR), I'm discovering that the video appears to playback at 50% yet the audio is good. The video was shot 1080p at 29.97 and that is what the project is set to. The video plays normally from Finder, but when brought into Premiere, it's horrible. As a side note, it imports and plays just fine on a dual 8-core Xeon. I guess that's what I'll be editing on for now, but I was excited about what the Mac could do!
Sorry about the poor experience. Can you please send a screenshot of the media properties panel in Premiere Pro. It will help us understand if Premiere Pro is misinterpreting the media file. Also, is the media playing properly in the Source monitor? Let us know, we're here to help.