Slowing down through aid station?

Use good judgement.  It's usually unsafe to fly through an aid station at 30 mph.  Someone could accidentally swerve while reaching for a bottle, or drop a bottle that rolls in front of you, or any number of other unexpected things can happen that could cause an accident.  You don't lose any significant time by slowing down and riding cautiously in the congested area.

With regard to slowing down: I've seen a lot of problems at aid stations when less experienced riders slow down on the far left more than they need to, and bozos bomb through screaming "on your left! on your left!! on your left!!!!"   So make sure that you're not blocking when you're slowing down.  It's a judgment call, and can be hard to get right. 

And then there are people wobbling like crazy leaving the aid stations, trying to stash bottles, drink something, unload trash, etc.  And if it's an out-and-back section of the course, you have oncoming traffic at full speed.  It can be downright dangerous.

Sometimes inexperienced volunteers can cause problems.  At IM Los Cabos this past March, no matter how far I went left, the teen-age volunteers ran out to try to meet me.  Bike or run, it didn't matter - I went left they chased me out, I went further left, then chased me further out, often getting in the way of other runners and cyclists.  I learned had to enter every aid station shaking my head left and right super visibly, saying "no gracias, no gracias" over and over.  They were super eager to be super helpful, and that's OK but there are limits.  I've seen volunteers chase people out rather than hold their line, but nowhere as much as IM Los Cabos.