I've noticed that Windows update is slow on some of my 4 home PCs, all Win 7 Pro x64 - it's particularly slow on the two wifi-connected PCs, fairly normal on the 2 wired PCs.
It seems to take hours to get through the download, which sits on 0% for ages. Patience is definitely a virtue! Network activity is suspiciously low and sporadic during the download phase of Windows update, but I don't know why and haven't investigated.
I don't know if the slow updating is related to Win 10 / GWX, but I have taken a machete to GWX and eradicated all traces of it in the registry and system files and the wifi connected PCs are still slow.
One still has to be very careful to not be on automatic updates and to read the details for the optional updates to make sure they are not related to GWX.
SD