i don't think you're alone. the reason i had to step away from public rp is because the people i found myself on the site(s) with posted daily & some expected high turnaround too.
some advice to counteract your dilemma is that i suggest joining multiple sites at once or participating in a lot of threads on the site(s) you're on with different people. it might seem chaotic but if you can organize it in a way that works for you, i believe it would give you the posting volume per day that you desire.