Hello,
On Moodle 3.6+ (possibly others) for 'all themes' that have blocks. I've discovered what I believe is an inefficiency in the block rendering process with the method 'blocks_for_region()' which is in '/lib/outputrenderers.php'. It is called by the 'blocks' method which is called by a theme to get the output for a given block region. So have raised MDL-64818. Please do comment / vote / peer review my findings.
Gareth