I'm getting a thousand messages like:

[04:03:38] [Server thread/WARN] [Bookshelf]: The ASM has not been initialized, there is an error with your setup!
[04:03:38] [Server thread/WARN] [Bookshelf]: The ASM has not been initialized, there is an error with your setup!
[04:03:38] [Server thread/WARN] [Bookshelf]: The ASM has not been initialized, there is an error with your setup!
[04:03:38] [Server thread/WARN] [Bookshelf]: The ASM has not been initialized, there is an error with your setup!

The server uses Telepads 1.7.10-4 and Bookshelf 1.7.10-1.0.15. Those two mods where added to the modpack and since then clients have gone really down in fps, and the server logs are growing at an alarming rate.

Any idea what's originating it? How can I fix it?

Thank you for reporting this issue. We have already been made aware of this issue, and will be patching it within the next 24 hours or so.

I can confirm that this change did fix the issue you have described. A new version of Bookshelf will be released within 24 hours. You will want to use version 1.1.x or newer.

It will be released when @Darkhax returns. There were a few other problems that have now been fixed.

Thanks! That bug was a show stopper, and you really fixed it fast.
Good work!

Version 1.1.0 has been released on CurseForge. It contains the fix for this issue, and several others, along with some new features.