it's the blizzlike spell delay system
there actually is a spell delay system on retail
there also is the so called custom lag tolerance, allowing lagging players to hit from more yards than not lagging players. This compensates some of the disadvantages of lag
If there was no lag tolerance, melees would be unplayable.
If there's too much lag tolerance, melees could hit you from 30+ yards when lagging.
The current system is the best regarding the blizzlike spell delay.
However of course server lag at high population still is an issue here, which can't be circumvented as the server simply is too full. You will never get a 100% lag free server with that much population