I wasted time with some long hour, large data size projects with low RAC

No time is wasted when crunching for science. And each project gets its own share of GRC rewards, so it doesn't matter if the project awards low credits - you get your rewards proportional to your RAC share, not according to your absolute RAC value. Meaning a project can award only 1000 credits per day - if you get 500 credits, you'll get half of the magnitude and half of the GRC rewards.