I've only noticed lag when we go there and populate the dimension. Load on CPU and memory is constantly on low numbers, like 12-20% load. It's the same problem when other dimensions are generated, but only when they generate and later the lag is out. I've discussed this with my friends and were more thinking of it as a disk issue as the process of generate overloads the disks. But I'm not 100 % sure.
If you are running sata HDD's yes you will find problems with your disk. Even normal MC will sometimes cap you out on your HHD's. The IOP's from a HDD are at max 200 IOP's. Compare that to a SSD that has over 100.000 even up to 200.000 IOP's. And since minecraft is writing and rewriting multiple worlds, player files, and much more, your IOP's will cap out VERY fast on a HDD, even 4 HHD's in raid 0.
(IOP's is a term reffering to Operations Per Second. )