Several monsters (for example, the Darkmantle) have abilities that cause their victims to be unable to breathe. However, when this happens in-game my characters just shrug and say, "whatever, I can hold my breath for 4 minutes." The battle is long over before suffocation becomes an issue. Similarly for situations like running into a AOE (such as a Stinking Cloud where breathing would be ill-advised.
Are there any mechanical disadvantages to being unable to breathe, before you get to the point of suffocating?
I know the rules for Suffocating from PHB p. 183 -- the question is, is there any mechanical penalty to not breathing during the minutes that you can hold your breath.
It's an interesting question as to whether, for example, the Darkmantle attack allows you to "hold your breath" or moves you directly to "choking" but that's really a separate question. What I am really asking here is, if you are holding your breath, as per the game rules on PHB p. 183, can you fight at 100% efficiency? Or should, say, disdvantage apply to attacks rolls, movement rate be reduced, etc.?
Q: If you can't breathe due to a monster effect (ie Darkmantle) are you immediately suffocating? Or have Con Mod+1 mins?
A: You can hold your breath.
(See Looking for play tested house rules for combat while holding your breath for a follow-on question about whether anyone has any good house rules for this situation.)