While using a screed is a slow process, troweling with a power trowel can be completed quickly. The rotating blades will move fast, so you have to move somewhat quickly too, watching the concrete to make sure it is smooth before moving on. If the blades hit the concrete too much or if the blades are pitched at too much of an angle, you will leave ridges and bumps that will have to be troweled again or fixed later with a grinder.