Thank you ! I just realized that I’m not using a 64bit version of Raspian while my rasp is 64 bit compatible.
Exactly. The 64 bit version is considered experimental and not advertised on raspberrypi.org.
Larynx is about 3 times as fast. Synthesizing 2.5 seconds of audio took about 5 s on the 32 bit OS and now takes about 1.5 s.
So far I have found no problems with the 64 bit version. Actually there was one: libspotify doesn’t work and I had to replace it with a librespot-based solution.