Excuse my not having posted on the forums before, but I've had some interesting experiences with FastCraft lately that I thought warranted reporting.
I recently updated my modded server and clients from FastCraft 1.9 to FastCraft 1.19, and the results were very strange. My FPS dropped from around 30-40 to 15-20. As well as frame dropping, I also experienced very strange, slow chunk loading behavior where the chunks were loading in in horizontal bands and every other row of chunks in the direction of travel was slow to load. If enough terrain was crossed, this would lead to the player walking into a world hole, which would not load in until relogging.
To summarize, the main symptoms were very slow chunk loading, reduced frames, and as well I also observed the game seeming to have performance issues on my machine, which was surprising. My machine is a Sandy Bridge-E hexcore with an SSD and 32GB of Ram, 8 allocated to MC. My server is professionally hosted with 3 GB Ram. I might add that my ability to track performance on the server is limited to observing the CPU and RAM allocation, with RAM sitting almost constantly at 100%.
My guess is that there is perhaps a mod or a mod interaction that is creating such a slowdown that 1.19 is revealing. However, these problems disappear upon downgrading to 1.9, again on both the client and the server. I thought perhaps since you would better know the differences between the versions of FastCraft, I would ask before trying to gather more information. I'm totally willing to provide more information if you'd like, though I'd need to know what you need.