Explode creates an absurdly high number of bullets (don't quote me on this, but I think it has enough for 12 loops of 12 volleys, or, in other words, 144 volleys of 3-5 bullets each. I'll need to find out where I saw those numbers, though...), and that amount produces INCREDIBLE lag.
However, while this many are produced by a single Explode object, with one Explode object you will never have this many on-screen at the same time, because some bullets die prematurely, and others leave the edges of the screen. But when you add more Explode objects, you increase the rate at which bullets are produced, and thus the bullet limit is actually approached.
At two Explodes, I get a slight, almost negligible lag. At three explodes, I get noticeable lag. At four, it slows down more.
When I have 9 Explosion objects, the speed eventually slows to its slowest possible (if you wait for enough bullets to appear), where it will take a whopping 5 seconds per frame. At about 75 Explosion objects (3 horizontal rows), it starts at that speed (not counting the first frame, before the bullets appear).