Spotting is caused by dirt being left to dry without it being rinsed off the glass,the common place for it to spot is the top right and left corners of the glass or a line down the middle under a top opener.
The better the flow and the hotter the water reduces this massively,cold users disagree I don’t want to get into hot over cold but it is a fact and the hotter the weather the more a hot system comes into its own.
The first thing you notice when switching from cold to hot is a huge reduction in spotting if any,another thing you’ll hear people say is that the frames look cleaner-whiter with hot 9 out of 10 that’s what is giving the trouble dirty frames or seals.