Did GCD change in macOS 26

Some users of my Mac app are complaining of redrawing delays. Based on what I see in logs, my GCD timer event handlers are not being run in a timely manner although the runloop is still pumping events: sometimes 500ms pass before a 15ms timer runs. During this time, many keypresses are routed through -[NSApplication sendEvent:], which is how I know it's not locked up in synchronous code.

This issue has not been reported in older versions of macOS.

I start the timer like this:

_gcdUpdateTimer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, dispatch_get_main_queue());
dispatch_source_set_timer(_gcdUpdateTimer,
                          dispatch_time(DISPATCH_TIME_NOW, period * NSEC_PER_SEC),
                          period * NSEC_PER_SEC,
                          0.0005 * NSEC_PER_SEC);
dispatch_source_set_event_handler(_gcdUpdateTimer, ^{
    …redraw…
});