The files can be a lot of small ones or one great big file. Typically users work with a lot of small files. The limited reads and rights refer to the fact that the speed at which data can be read from an ssd or written to an ssd are limited by the SATA 3 connection.
Think of it this way - You have a fast automobile that can exceed a speed of 100 miles per hour (160 km per hour). Unfortunately the absolute maximum speed limit in the city is only 50 miles per hour (80 km per hour). The speed limit causes the automobile to operate at a lower speed. The SATA 3 connection is like the speed limit. The SATA 3 connection limits how fast data can be read from or written to an ssd.