The Ardour package version for Debian Bullseye is 6.5.0+ds0-1 (6.9.0 will come with Debian 12)
I tried to build and install Ardour 6.9.0 on a fresh Debian 11 : all seems to be ok (even if there are some warnings)
Here my method for those who would be interested in:
It was on a low conf PC, 4GB RAM without SSD. Maybe that could be checked with strace (for instance, I think about LD_LIBRARY_PATH : lib paths are nearer in /usr/local, and there are less system calls to reach them).
The “problem” is that to uninstall you have to run ./waf uninstall with exact the same configuration and version that you have installed. If you meanwhile git pull or re-run ./waf configure ... with different parameters, uninstall may not work or leave some files behind.
This is known to have caused issues in the past when different versions have been mixed.
That will be ok for me because I don’t use the git way : I download the zip of each version and I keep it unzipped in a directory like “Ardour-6.9.0”, so I keep the configuration and I can properly uninstall.