Moodle checks if file $CFG->dataroot/climaintenance.html exists on each request.
If it does, maintenance mode is enabled.

However, on slow filesystem (like NFS mount) this check may be slow - and on systems with many requests, it will affect the performance of Moodle.

The check is done in:
https://github.com/moodle/moodle/blob/master/lib/setup.php#L322

if (file_exists("$CFG->dataroot/climaintenance.html")) {

I suggest to create new $CFG entry and make the location of climaintenance.html configurable, with the default pointing to $CFG->dataroot/climaintenance.html. This could be configurable in config.php only, with corresponding documentation in config-dist.php.