Massive lag + crashes on join (depending on who joins)

Also affects 41a/b , does not affect 39b

When a 'laggy' (lets call it that) person joins the server the server starts lagging and everyone gets disconnected. When a 'unlaggy' person joins everything works fine.

one or mulitpy unlaggy perons = fine
one laggy person (alone / and not alone) = crash for everyone on server

i have no clue what causes this. (started with w41 snapshots)

Edit:
i probably formulate that a bit fishy, i just didn't know how to formulate it other way. a 'laggy person' in my description is not a person with a high ping (the same persons have no problem when the server is running on 39b) they are ppl who generate (w41+) a huge amount of SERVERSIDE lag on joining (again, on snapshot 41+ including the pre-reslease).