Yup, can confirm that function is expensive. A lot of time was spent tweaking the pond generation to get away with as many lillypads as possible, whithout performance drops. I think you can still get frame drops if part of the map generates unusually dense, but I haven't had that happend in the last week of testing.
The game wouldn't have happened without this function. Using sprites just wasn't the same.
I will link the URL to your function in the discription of the BBS version I'll be posting monday.