If Optimise Storage is turned on, why do videos in iCloud.com buffer?

As above. A small circle appears in Photos on iCloud.com in individual videos in bottom right of screen that gradually fills up, then video will play. Because I keep the originals in the cloud and especially when I open them in iCloud.com, why do they buffer here? They buffer on my devices also (but not on laptop) but I cannot understand that as the originals are not on my devices but are in the cloud, so where could they be downloading them from? Is it something to do with internet speed, ie, speed to the router, or is it due to poor connection between the router and where am sitting with the device? Do I need faster internet speed to the router or a booster in the house? We can get FTTP, fibre to the premises but faster speed to my router I think, does not mean good connection from the router to the device. Getting FTTP is quite messy and more expensive. Please advise.