I considered joining a great looking advanced RP at one point a couple years ago, but decided against it when I noticed how infrequently players post as a whole in the subforum. It's worthy to note that, despite how much effort went into the lore and world building, the RP died shortly after.
I've been part of a roleplay called The Elder Scrolls: Fruits of Contention for a few years(!) now. It often moves at a positively GLACIAL pace and the GM was even forced to prematurely enter Chapter 2 with a little timeskip to grab everybody's attention again and resume posting. But it works. We can go weeks between posts or take months to resolve a single encounter, but we're all competent writers who have long-term plans and visions for our characters and we have an extremely dedicated GM who similarly has a really expansive vision and fleshed out plot in store for us.
It is, by Guild standards, practically unique in that regard. I can only think of Ellri's Star Wars RP and the long-standing Create-a-Hero RP that are in the same category of size and age.
It's possible. You just need to work at it and get lucky with a dedicated and capable group of players. There are ways to make yourself lucky, though. It sounds stupid but pick the right fandom! TES and WH40K roleplayers are pretty hardcore about sticking with things, I've noticed.