The point is the same setup plays full screen flawlessly on same-era (late 2010 IIRC) regular MacBook, which we worked on until my partner inadvertently updated to High Sierra. The project file is stored on internal disk, while the footage is on NTFS USB 2.0 HDD(s). Actually the one used with the MacBook Air in question is arguably much less cluttered/fragmented than the one used with regular MacBook.
The Air plays acceptably on external monitor in nearly-fullscreen size (but not in Fullscreen mode), while using Fullscreen it’s just spectacularly frozen.
Sorry I didn’t add it, forgot while my post was in moderation. The sequence and footage date is as follows:
1920 x 1080, 25 fps, Apple ProRes 422, aspect: square, field dominance: upper (odd)
Sequence settings: HDTV 1080i (16:9)
Regards,