Did you turn on overdrive? And eventually "scheduler in audio interrupt"? Maybe you should.

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I tried overdrive alone and then with scheduler in audio interrupt and there was barely any difference. Thanks anyways Patrick.

I didn't think you could change the playback tempo of [seq] after you started playing. Unless you're using a "next" message system. Maybe try [detonate]?

Though playing back [seq] at the default tempo (1024) should play back your file rock-solid. I don't know what's happening there. My laptop is a bit stuttery with this sort of thing, but my desktop is totally fine.

Try some other scheduler settings as well, like less slop and whatever else might help with timing. I think the only pretty-much guaranteed way to get solid timing is to use audio objects to generate the timing, with Overdrive/Audio Interrupt enabled. That's worked great for me. So... and this is a general question going out to any and all wizards... does changing these settings make any difference when using something like [seq]? And if not, how can we ensure proper timing? I realize that things like [metro] will drift, but [seq]? Seems like it should have additional timing tightness built-in, if that's possible. And I know there's seq~, but that's not for MIDI files.

Using [seq]'s timing does mean that one cannot timing while it is playing but I was using the [tempo] object to send "tick" messages. I found it strange that both [seq]'s timing and [tempo]'s seemed off. I will experiment with [detonate] and let you all know my findings. Thanks for the help and please keep it coming.

Bad news. On my laptop at least, [detonate] is not output MIDI in straight regular tempo. I will try my iMac later but I might start looking for another solution.