I really like how enemies have chunks getting knocked off of them when hit. The idea of body heat being substituted for health was a nice way to consolidate the mechanics and convey that the creatures kill people by freezing them.
I didn't notice any lagging issue on the browser but I have a fairly beefy computer. I did have an issue with the mouse not focusing back on when I clicked back into the game... It forced me to refresh the browser a couple of times early-game, but I don't really consider this a big deal because it's a 6-day jam, the game is pretty quick, and I was still figuring out the controls/combat at that time.
I found enemies more satisfying to damage than to kill, and too easy to avoid... I was really missing some finality to them dying. The controls also felt a little clumsy and there were two occasions where I equipped the sword during/after changing the filter, entered the "sword fighting" state, was able to swing, but wasn't holding the sword.
The environemnt looks great with the style you were going for and the different packs blend together nicely. The bleak story ending also worked really nicely with the short format of the game!