When you begin playing CLM, it takes 1 second to plant/harvest 1 crop.
Harvesting more plots gives you Mastery stars, and with each Mastery star your earn for a crop, the time it takes to plant/harvest that crop is reduced by 0.1 seconds.
The RC shown below the star does not mean you will get that much RC when you earn your next star, but that you can buy the next star (and reduce plant/harvest time by 0.1 seconds) without having to harvest any more of that crop.
The text in blue says “Harvest X more Y plots for the next star OR GET IT NOW FOR Z RC”.