Well it wasn't instant death, I was exaggerating out of aggravation. But they were pretty much filling the room, and filling the small space where the switch was. So when I hit the switch (which had never before caused bad things to happen) and the room was suddenly full of fire and there was no safe space to dodge to, I wasn't really sure what the heck was going on. I figured out that they were destroyable right around the time that I died, which probably did only take 5 seconds or so.
I'm not sure I like the idea of a trap like that. It doesn't add anything to the game except for one lame death. From now on, of course I'm going to clear them all out before pulling any nearby levers. So that simply means that it's a 'gotcha' to a new player and it might as well not exist for a player who has experience with it. Why not something more interesting, like pulling the lever turns on a series of intermittent flame jets that you have to dodge on your way back out of the room? Admittedly, that's an idea as old as dirt in the world of platforming, but at least it's some sort of challenge.