I've dealt with Mystcraft multiple times on servers, and I can say that Slind is right.
Like many other server owners who care for the stability of their servers, they disable Mystcraft ages, but not the mod entirely, since players still love the link book.
MOST server lag occurs during world generation, which occurs when players explore new areas. It's not as bad if they exploring new areas slowly (via walking/sprinting), when coming from already explored areas.
However, it's the random teleportation to new areas, and moving quickly (boots of traveller, flying, etc) that can lag the server as it tries to generate new chunks only to have the player leave that chunk and demand yet another new one to be generated as they whiz through the world.
Because of this, my personal recommendation is to, yes, allow players to keep the link books, but, get yourself some sort of chunk/world generating mod onto your server and run it during low population times (like in the wee hours of the morning)
My personal favorite is a mod called
Admin Command Toolbox. Created by the same guy who made Opis, it's a mod you simply put in your server's mod folder (players don't need it to connect to your server) and run a command to generate chunks.
When chunks are already generated on a server, they simply go dormant/unloaded until they are needed again, such as a player getting with view distance of them (another thing that helps with server exploration lag - lower view distance in server properties)
I personally schedule a command to be ran through my console's event creator - somewhere in your console that lets you create scheduled events/commands. Just do the command for generating a dimension (overworld and Twilight Forest are the 2 most important ones - end and nether are negligible), have it run at, say, 4 am, and run another command to stop the generating at, oh, 10 am. Doesn't' matter about the times, just so long as you run it when players aren't online, and STOP it some time alter on... typically, when this mod is generating chunks, it will cause players to not be able to connect (times out), so be sure you cease generating around the time when players get online for the day. Do this each night for a week or two for the overworld and Twilight, and boom, players can fly around all they want and not cause a hitch. Unless they explore out to like 100,000....
The other neat thing about this mod is that it has a purge chunks command, which I run every 4 hours. It simply purges all unused chunks, forcing them to unload from memory. Minecraft isn't the best at unloading unused chunks, and you'll get a ton of chunks that are sitting there loaded, entities and all, even if a player is no where near it. Seriously, after running the command to purge chunks, our server goes from having 2000+ chunks loaded to under 300.