It's compatible with the latest CMSB.

I tested the Load Monitor with CMSB v3.04 and it works fine.

Do you know what page triggers the "slow page" notice?
If you're expecting a page to have a long run time, ie. cron.php, you might need to override the globals:

// NOTE: Add this code to the top of any specific pages where higher memory/runtime is expected (eg: cron.php)
// ... Make sure you add it after viewer_functions.php or init.php is called.
/*
// Load Monitor Plugin: Override/set logging limits
$GLOBALS['LOADMONITOR_MAX_SECS'] = 5; // pages that take longer than this to load will be logged
$GLOBALS['LOADMONITOR_MAX_MEGS'] = 10; // pages that use more memory than this will be logged
*/

If not, the issue could be on the page that triggers the notice or the page needs to be optimized.

Is it always compatible with CMSB3?

http://www.interactivetools.com/forum/forum-posts.php?postNum=2237150

I receive this error permanently:

E_USER_NOTICE: Load Monitor - slow loading page: 1463236635.85s
.../loadMonitor.php (line 49)

Thanks!

Djulia