Don't do "Pick every unit... " action
Just use, "Create Dummy unit" and "Order the dummy to War Stomp" around the position of Caster
Change War Stomp's Area of Effect to 600
There you go
EDIT:
I've uploaded a test map
By doing my trigger, it saves performance and lag-free
Cause', your trigger involves creating a dummy unit more than one, while mine, it just use ONE dummy unit at a time and it saves you from lag while casting this spell