What impacts stocking times?

From the Wiki:

At the cost of 3 Tower Bux, the player can permanently upgrade a commercial floor. Each level of upgrade does the following:
- Adds 75 Stock to each product
- Adds 45 Coins to the cost of restocking each product
- Adds 5 minutes to the restocking time of each product

Upgrading is thus best used for low-stock floors as you can boost their stock by the largest relative amount and increase their stocking time to something more sane. Type of floor is irrelevant, and there doesn't seem to be a max floor level either. Just upgrade low-stock floors to the point where you can comfortably restock at your own pace.

Note Dream Job stocks are actually boosted by 150 stock, the 75 stock boost is applied before the Dream Job multiplier.