Heartbeat interval resets to 15sec when enabled?

When I enable this plugin, the heartbeat interval gets reset to 15 sec no matter what I do to alter it. I’ve tried using the Heartbeat Control plugin as well as adding custom code to my theme’s (Thesis) functions.php file, to no avail.

Anyone know where I can find the code that would affect this, since we’d like to stay with the classic editor for the time being rather than switch to gutenberg, but the 15 sec heartbeat is too much for our servers to handle and causes a lot of problems.