I have a heatmap with 61136 features. This takes Chrome 17 Seconds to calculate each time the heatmap gets recalculated. Almost all of the time is spend in this method:
Looking at the code, and taking some snapshot values, I'd guess the inner loop code gets called ~21 Billion! times.
Not sure what this method actually does, since I can only look at the compressed code, but it seems very inefficient.