I think Windows does suck up a lot of CPU in relation to tablet OSs and maybe even in comparison to Windows RT.
I do agree with poorly designed websites but also people can have dodgy internet too.
My wife who is the one who is the real owner and user of the Surface 3 said last night "my email is slow, a circle is going round and round". It wasn't there when I looked but it does show that the Surface 3 will bog down, for whatever reasons, much more often, than say the SP3 will. On the other hand, as you say, it is faster overall than the Surface 2.
I think the Surface 3 is cruising on the lower edge of acceptable performance, as you would expect with eMMC and an Atom processor but it is impossibly small for what it does and is lovely. If anyone has ideas on the relative power draws of eMMC versus an SSD, I would be interested as the decision to use eMMC could be driven as much by power considerations, as cost.