Of the one RP I'm in, I have gotten to a point where I've JPed with just about everyone lol, and I've done posts where it is not 2 or 3, but 4 people at once. JPs really tie tie players together if done well, and while the slow down can be a little annoying, it's usually fixed with some occasional nudges.
The problem with forum post interaction (FPI?), and this is a problem that I had even 3 years ago when I joined this forum, is that you can't have conversations without pointless filler to pad your post for the limit. Additionally, the interaction can take longer because you make the post, tell the person, they make their post, and then you make yours. In contrast, a lot of my JPs are, "Get in the doc" and then anodded hour or two of everyone adding sentences and character reactions. Things go fast, and I've noticed that our characters can complete tasks and events quicker.
Check out some posts on Dust, I havent done a solo post since something like my 3rd post. Admittedly, our posts are depressingly long, but that doesn't really feel like much of a bad thing to me.