There's a check to update the session time only once a minute, but this is ignored for AJAX requests which results in unnecessary UPDATE queries along the lines of:

UPDATE phpbb_sessions

SET session_time = 123

WHERE session_id = 'abc';

https://www.phpbb.com/community/viewtopic.php?f=556&t=2461861
https://www.phpbb.com/customise/db/extension/mchat_extension/support/topic/190116