Probably no big external dependencies or any assets. It looks like something full of handwritten code, down to the self-rendered UI-elements. Also it's written in C/C++ and assembler, so there is no unnecessary bloat shipped out-of-the-box.
How big are the project files these days? In the .aup3 format all source material was rendered into one big pile, so with lots of "raw tape" (e.g. for a radio show) the session files became huge. It was annoying to transport an 1.5 GB session file between two machines even via network.
Reaper and Pro Tools (and many other DAWs), in contrast, had the source material in one folder and the session file essentially just documented the edits. These small session files could be exported even via email.
The .aup3 format was one of the reasons why I decided to ditch Audacity3 for radio editing back in the day. And, it used to lag heavily when I had a project with several hundred cuts done via "ripple editing". All this was years ago, though.
I concluded that Audacity v1.2.6 was the high point of this software and, indeed, tried to go back into using that one for a while (in 2019 or 2021 or so)! It was fast enough, very simple, young, raw, robust, but it still had its early 2000s open source enthusiasm charm. :)
Just an educated guess, but perhaps there are optional dependencies missing in the arm64 version. Some programs are still broken or perform badly on arm64.
