Google Translate:
First of all, the ActivityPub system is not suited to something like Misskey, which flows at lightning speed.
It was originally designed to connect blogs, small-scale SNSs, and wikis.
How can it handle the TL hell where tens of thousands of requests fly in per second?
It’s based on the idea that it would be nice if various small services could send each other updates, so it’s quite costly.
I’ve used Calkey and Sharkey and they quite similar to Mastodon but with a prettier UI. I don’t know Tumblr so I can’t say.
Tumblr was more long form than original Twitter and you could more easily post music to it and it developed a rich quote-post and tagging culture. Not sure what happened there after I left.
You can post longer on *key that on Mastodon. I don’t use a lot of microblogging so I can’t tell you much more. Maybe you should look at an instance of each to compare :
Mastodon.social
Sharkey.world
Oh ! I just learn on it about the CloudFlare. I couldn’t figure out why this one site was not working. The fediverse is still going strong. Life is sweet for our little corner of the internet.