im confused as to what you are talking about
current lag especially in pandaria can be due to world bosses and open world combat from max level players due to all of the procs
as for nyalotha its much the same where you can have more players (its farm and prepatch so a lot of people are returning) with more procs now than before
as for phasing and sharding in the previous expansions the thing is theres less phasing due to a lower pop