If you're doing Penumbra key quests, the time it can take to attain 35 Fragments of the Source can be painstaking.
However, I was in the spirit room of the redeemed temple looking for spirits for my shade, and managed to fill 6 bp's of fragments in about 1.5 hours.
So there you go, if you want to get your 35 fragments quicker, head to the temples in Penumbra and get your fill.