There is no way to “speed up” federation. Lemme doesn’t backfill comments, nor posts, and will only receive new content if at least one user on your instance subscribed to a specific community. This is because the federation process (beyond the initial search) is not a pull model in which your server requests information from other instances and stores it; but rather it operates on a push model in which you tell larger instances that you’d want new information (by means of having at least one user on your instance subscribe to the community) and they push that to you. Once that subscription is made, only new contents created afterwards are pushed to you, as fast as the other instance could. This as you can imagine puts a lot of stress on the larger servers as they’d need to update every interaction to thousands upon thousands of subscribed instances. “Seeding” content en mass (making dummy accounts to subscribe to everything) makes it worse because now the other instances are pushing entire community’s new updates to the new instance where no one ever touches it.

Instead. It’s probably best to just subscribe to a small handful of communities that you actually care about and intend to read, so only content you will consume gets pushed to you. Then invite like minded people to join and gradually, they too will add more that they’d actually read, so your instance isn’t just sucking the resources out of the fediverse at large but not actually using it.