The minutes and hours of Rosh Hashanah are extremely precious and should not be wasted. This is a time which is more elevated than the rest of the year, both in quantity and quality, in the same way the brain is superior to the other limbs of the body.
It is appropriate to recite Tehillim during one’s spare time on Rosh Hashanah. Some people endeavor to complete the entire Tehillim twice. The number of chapters recited (300) is the gematriyah of kaper (atone). It is better to recite less quantity and have more kavanah (concentration).