It turned out that - out-of-the-box - a production environment is configured in ManageIQ.
Unwittingly, I kept on using that profile, and finally found out what was slowing me down.
If you incidentally start developing using a production profile, the easiest way to have .haml files activated on the fly, is to change the 'config.cache_classes' property from true to false in file /var/www/miq/vmdb/config/environments/production. rb.
Ofcourse, the correct solution would be to configure a proper development environment (setting environment variable RAILS_ENV in file /etc/default/evm from production to development), but that requires some more tweaking, e.g. since a switch of database is involved.