I understand what you are trying to achieve, but he isn't really designed for that. He takes only 9 seeds at a time because he is working based on a task queue. We can make him take all the seeds he needs at once, but that only makes a difference during the first setup (like you have here) when the farm is completely empty. When everything is running, it doesn’t really affect efficiency, and you won't be worried about speed.
You said it took around 20 minutes to finish planting 42 planters, but please keep in mind that he also watered them, added fertilizer, and then planted seeds. Watering is the slowest part, and since you have sprinklers, that won’t be an issue in the future because sprinklers will keep water at 100% so the farmer will stop watering on his own.
The design goal here wasn’t ultra-fast farming, it's more about fully automated process so you don’t have to handle this farming routine yourself. If you just leave him working, he’ll get everything done for you as soon as he can
About performance issues, did you try to set "Controller tick" to 0.2?