Couldn't for the life of me figure out why my server had suddenly become so laggy, as I added minecells along with a number of other mods to my modpack at the same time. Discovered that minecells keeps all of its dimensions loaded, treating them similarly to the overworld with its spawn chunks, at all times. This adds significant lag to the server, and as of yet, I cannot figure out a way to get this to stop, so I was sadly forced to remove minecells from my pack.