The current render distance should be something like 4 chunks. So whenever you walk around there are 81 chunks loaded around you, make that 5 and it becomes 121 chunks, make it 6 and it becomes 169. The amount of chunks increases exponentially. And thus the load on the server increases exponentially as well. Normally this isn't much of an issue, but the anti-xray hooks into that chunk loading, and thus increasing the amount of chunks drastically increases the load on the server. For a server ECC's size that's not feasible.
Erm, iirc, aren't rendering and "loading" different?? Ex. It may be that a 4 chunk radius will render...but I believe that only 2.5 chunk radius will "load"/update. That may decrease the load u guys are talking about so increasing the render distance may not be as much of an issue.
also, iirc, if you increase your render distance then your client will keep the chunks loaded, so just make it like 30 and load the chunks you need, lol.