This is a known issue that is about to be resolved ( Fix tractography rendering performance on macOS by daljit46 · Pull Request #3197 · MRtrix3/mrtrix3 · GitHub ) in the upcoming release.
For the time being you can use the workaround described here: https://community.mrtrix.org/t/issues-with-pseudotube-rendering-on-apple-m1-macs