I personally would wait until it is bigger and then eat the thinnings. It grows so fast and bolts so quick it's not really worth transplanting or thinning. Just harvest as soon as it's big enough to pick.

Thats what I read to do when thinning out to wait and eat them. I was just wondering if being so close together would slow down the growth.

My arugula self sows and this saves me the trouble of planting the seed. This plant does fine even in crowded situations.