I'm practically a fixture on Lemmy, and I view everything sorted by newest comments so I see only new posts and posts actively being participated in through replies and I'd say it's only slightly less active than Reddit appearance wise. Surely there is less things being posted over all, but I can just refresh the page every few seconds and get entirely new posts almost every single time, barring a few hours in the middle of the week.

I know that someone has a statistic site for Lemmy that could actually show you exactly what you wanna know, but I haven't saved the URL and don't know it off the top of my head.