Collecting harvest is slow if you click each item and wait for the pick-up animation to play. Game time pauses during animation, so there's no worry there, but the whole process is very slow (real time spent per crop gathered). Is there any way to collect multiple veggies per click like upgraded tools do?
The fastest way I've found to gather non-scythe crops is to select something in my inventory that isn't a tool and can't be eaten. For example, a jar of jelly.
While that's selected, I move along the crop rows while holding the action button and moving my mouse across the harvestable crops. Because I'm holding something that can't be used or eaten, there aren't any actions to interfere with crop gathering. I can harvest two or three rows at a time this way.
The only thing that still gets in the way is animals, which get pet or examined by the action button if they walk in the way. You can get around this by fencing in your crops so the animals can't get in there. (They don't actually eat crops; the fencing just keeps them out of the way.) Or just harvest your crops after 5pm or so when the animals go back to their barn/coop.
One way I've found to increase the speed of gathering items is to stand close to a row of crops, press and hold the collection button and move the cursor through the crops. When the animation will stop, you will need to repeat.
Basically, surround yourself with nearby tiles which have crops and swipe the cursor through them all.
Also, as I've found out, right-mouse planting seeds at the same time as gathering crops speeds up both, and the reach is farther somewhat. But this is a bit dangerous because the seeds may get planted on undesirable tiles, even if you didn't hold the cursor within them.