Thanks for the exstensive feedback! About the materials despawning: It was a choice we made to improve performance, as the amount of loot on screen was getting too large to handle. As for the collection time, it was to prevent simply building a fast city and just zooming past enemies to collect upgrades and out scaling everything. I think there probably is a better way to achieve this, but it's what we went with for now.
Thanks again for the review!