actually, not as much as you'd think. I've triggered a spell that acts as an opposite to yours (even though it's chance) and it doesn't lag whatsoever. Even if, say, 20 units are hit at the same time over 5 hits, it won't lag that bad. if you aren't too picky about spells i can post the template for my spell and you can mold it to work for yours.
The best bet for barrage is to have it already researhced. Also change damage to zero so it deals what your hero delas, also change area size. That should work because i used it for my spells and it works perfectly. I dunno why yours wouldn't. One thing for usre; the delay doesn't affect it. Make sure you take off the techtree requirements and set the proper parameters for as much as you can and try it again.