my Moto G had almost dying slow download speed from Google Play store. I read many threads. One temporary solution which I found in a thread within this forum, worked for me.

Used Menu->App option to reach to Google Play services listing there. Deleted cache & data from Google Play services followed by Google Play store.

My downloads improved rather got reset to what it was/should have been.

However I feel that this is a temp solution only. Google needs to do something to resolve this problem on all Android devices (namely Nexus 5/4 etc, along with Moto G offcourse).