I do the same thing. I use all as my home page so I can see new and interesting stuff.
I tend to block most 'ism and 'ism-adjacent communities, Foreign language communities, political communities, NSFW/Fetish, and a few highly specific tech/app/language communities that mostly only post version updates.
Bots. lots of bots. If your community is 90% bot posts/repost aggregation and I see 5-10 posts consecutively in 'all' because your bot just vomited up it's entire load in a 120 second span then I'll probably block it.