Posting Rates as Expectations

The post frequency of a pbp game tends to slow down to the post frequency of the slowest user. It's just the entropy of play by post - systems tend toward chaos.

One cause for the slow down can be that somebody in your group begins experience some IRL difficulties which impacts their ability to post. A second cause can be the game bogs down at some decision point or some transaction-heavy game mechanics, typically combat. A third is that a player might feel uncomfortable or realize the game doesn't match their expectations, and start to disengage. The first is inevitable, the second can be smoothed out to a degree, and the third can be partially addressed with safety tools and setting clear expectations from the start.

People tend to slow down as other people slow down for sequential and social reasons. Because actions happen in sequence, they might be waiting on the response from other players before they can proceed (ie: a conversation between two PCs). Faster posters might also feel social pressure to slow down. Maybe they want to be inclusive and wait to make sure other users are getting their fun, or maybe they feel guilty like are hogging the action. They might also just have missed something by accident and are holding up the game without knowing.

A GM can help reduce pressures to slow down (but never eliminate them). A GM can be like a good manager, vigilantly watching for bottlenecks. They might ping players on discord when they're needed to post. They might have an ooc or private conversations with players that are struggling to keep up. They can use safety tools to address discomfort when it occurs. They can foster an environment where people feel comfortable enough to talk openly about their likes and dislikes of the game. This can feel like a lot of work, however, and grinds GMs down and erodes their willingness to manage things.

Players can also reduce pressures to slow down. They can check the game daily (or at whatever freq. is expected) on their own volition without being prompted. They can build characters that use fewer transaction-heavy mechanics. They can clearly communicate when life's difficulties impact their ability to post (ie: being sick) and give permission to everyone to move forward and automate or ignore their character's actions. They can also be proactive in their communication about what they do and do not like in the game rather than passively disengage. And, they can be supportive and express gratitude to their GM for tirelessly keeping the ship afloat.

And the more everyone slows down the less social pressure the slow player feels to try to keep up.

Many of these are inevitable, but in light of the the original question:
Players should check the expected posting rate and not join games they know they will not be able to keep up with. I check and don't join games with too slow a post rate, and don't think it is fair when others ignore the '1 per day' and thereby make my decision making process invalid.

This is part of the social contract, the game advertises that it is expecting a particular rate, and players join based on that, it should not change without good reason. In the same location the game also advertises that it is using a particular ruleset, it would be no less rude to change that after the players join... not without significant discussion.

Does anyone here have experience with co-op / co-gm play instead of the traditional gm moderated? Seems like there is potential there to smooth over when a player goes through periods of inactivity. I also suspect the same of PbtA style games, where players drive a major amount of the narrative and there is less emphasis on turn order. I don't have much experience with PbtA games nor with co-op games yet tho, so I can't speak from experience.

I have only co-oped with select players I already know, so I think that is a bigger factor on how well the posts and post rate management went than the fact it was co-op.
When I play regular games with people I know well there is very little trouble with this, it is mainly 'strangers on the internet' that are a problem. If we have an existing --possibly outside of the game-- relationship (and dialogue) then slowdowns are more easily handled and worked around.

Another factor here is that co-op games tend to be smaller, and, as mentioned by a few above, the size of the game can really affect the regularity of the posting (lowest common denominator and all that).
The lack of turn order (called 'initiative' by some games for silly historic reasons:) does make PbP a lot easier.
Though the GM does now have an extra burden to make sure everyone gets their turn in the spotlight since the order no longer does that for them, but it is worth it and they have to do that anyway outside of 'combat' --or wherever the turn order is enforced.

Though, I have also seen games where the enforced turn order kept things going, and games where the lack thereof meant everyone stood back and waited for everyone else to act.

Proper communications helps.

Many good points made all throughout this discussion. Many, many factors in play. But I think everyone agrees communication is key. Unless somehow unable (you have lost the use of your digits, all electronic communication has gone down in your area, etc.), one must do one's best to communicate.