Some of you may have noticed that federated actions are slow to synchronize between Lemmy instances. This is most likely because of the setting "Federation worker count" under /admin. It determines how many federation activities can be sent out at once. The default value is 64 which is enough for small or medium sized instances. But for large instances it needs to be increased.
Grep the server logs for "Maximum number of activitypub workers reached" and "Activity queue stats" to confirm that this affects you. For lemmy.ml I just changed the value to 512, you have to experiment what is sufficient. The new value is only applied after restarting Lemmy. In my case changing the value through the website didnt work (maybe because its overloaded). Instead I had to update local_site.federation_worker_count directly in the database.
Edit: I had to increase the value to 160k for lemmy.ml. Now the stats arent getting logged anymore, so Im not sure if the pending queue is still building up or not.
I did notice that syncing has been slow lately, hopefully this does fix it.