Auto kick players who are default, starting with the lowest tick count if the server tick rate goes below 10 ticks per second.
this would be my solution to the lag. limit the number of players, with a sort of white list that lets the paid players priority to the server.
or getting a ton more ram and using a ram disk to store the game so that load times are improved. warning this would subject the server to lots and lots of bad things if the power went out. but ram has much faster read and write times than SSDs. however I guess i do think some ram disks have a back up to hard drive feature. so maybe not too much would be lost in a power failure.
however i am not a computer guy. nor have i even attempted to look at hosting minecraft servers so i have no idea how practical theses suggestions would be. and please don't take them because i have no idea what the far reaching complications could be, you know the butterfly effect and all that! maybe a ram disk will use more power which will cause rolling black outs and the end of the industrial world!