You may be blocking the main thread or you might be flooding it with events.
I would suggest three things:
Grab a timestamp for when the timecode arrives in the CoreMIDI callback thread (see
mach_absolute_time()
. Then grab the current time when your main thread work is being done. You can then adjust accordingly based on how much time has elapsed between posting to the main thread and it actually being processed.create some kind of coalescing mechanism such that when your main thread is blocked, interim timecode events (that are now out of date) are tossed. This can be as simple as a global NSUInteger that is incremented every time an event is received. The block dispatched to the main queue could capture the current value on creation, then check it when it is processed. If it differs by more than N (N for you to determine), then toss the event because more are in flight.
consider not sending an event to the main thread for every timecode notification. 25 adjustments per second is a lot of work. If processing only 5 per second yields a "good enough" perceptual experience, then that is an awful lot of work saved.
In general, instrumenting the main event loop is a bit tricky. The CPU profiler in Instruments can be quite helpful. It may come as a surprise, but so can the Allocations instrument. In particular, you can use the Allocations instrument to measure memory throughput. If there are tons of transient (short lived) allocations, it'll chew up a ton of CPU time doing all those allocations/deallocations.