@ricard I don't want to speak badly about his host: masto.host is very competent, and very good at what they do, but ultimately that is the cost of shared infra, I'm afraid.
AFAIK @aral is running on a plan which provides him with 12 sidekiq threads, for $40 pcm.
I'm on a VPS that costs me $14 per month, and is running 25 threads comfortably - so more than twice as 'fast'* for less than half the money.
For $40 a month he could probably run a server that could sustain far more threads on dedicated hardware, but it would come at significant setup cost: Optimising sidekiq and Postgres for Mastodon is far from trivial, and that's part of what you pay for with a hosted service.
*) Obviously speed is about more than the number of processes, so please don't take this too literally.