In case anyone was curious, today I installed the Android 15 "March feature drop" update which includes a virtualized Linux subsystem called just "Terminal", tucked behind a default-off toggle in the developer settings. It's not stable yet: it crashed a couple of times.
It's stable enough that I compiled emacs 30.1 right on my phone, and compared the speed of elisp-benchmarks on my self-compiled emacs against the Termux version (without native-comp).
Result: _virtualized_ emacs is ~2x faster!
...and just to cover all the bases, here is a bit of background info about the benchmark and the specific way I ran it:
https://paste.sr.ht/~gnomon/72d1410af2a280b3c9da539b64f1bc4850faff57
(cc: @sacha )
@gnomon ooh that's promising. Does it have file access?
@sacha the VM is configured to map /mnt/shared/ to the Downloads directory, which is also accessible from within Termux at ~/storage/downloads/; it's awkward but it does technically work. I haven't benchmarked I/O performance through it yet, nor checked whether I can create shared sockets in it.
But I can definitely SSH from Termux into the Linux VM as long as I configure sshd to listen on a port >1024! Tramp, rsync, & git work; in principle sshfs from within the VM back to Termux should too!