Not only sorters but also semi-auto smelters, special casinos and such, I think there are ways to fix hopper lag without the reducing of the transfer speed, most lag is generated by hoppers that are uncovered and looking for an item to be picked up.
For that I would suggest slowing the "hopper-check" significantly. and reset the item transfer speed to the vanilla way. This does cause also problems but less so than the transfer speed.
I understand you want to keep the server less laggy, but because hoppers are behaving differently there is a need for bigger and bulkier Redstone to avoid using hoppers, and that again causes more lag.
I know the goal of the server is to keep the server as vanilla as possible, nonetheless I want to suggest this :
https://dev.bukkit.org/projects/sortinghopperit's a plugin that will remove a lot of hoppers from the server (not fix the lag they generate, most hoppers used atm are for sorters (at grinders and storage rooms) when we can replace 3 hoppers (for sorting 1 item) and replace it by 1 hopper this could reduce the lag.
