There was an issue with Blackmagic Decklink cards a little while back… though you don’t have one, it seems. It had to do with setting the card prefs (in AE) to faster previews, if I recall. Ah, here we go:
“If you have a Decklink card, try setting your video preview pref to “faster”, and/or matching the 16:9 or 4:3 setting to your 16:9 or 4:3 comp format … all in the prefs.”
You may have a smiliar problem with the Io … can’t say, sorry.