The CPU is usually much faster than any peripheral device. In fact, it would not be strictly necessary for the CPU to immediately return to the top of the loop after getting a byte if it "knew" that the device was so slow as to cause a significant time lapse between bytes. In this case, the CPU would not have to set the data accepted bit to 1, and in fact no data accepted bit would even be needed! The CPU could turn off the data ready bit when it snarfed up the data byte, and then go off and do some other work for a while. Eventually, it would return to CHECKCTL just about the time it expected the next byte to appear from the tape drive.

Is it realistic that the CPU is so much faster than the peripheral that it can risk doing other work while waiting for the peripheral? Often, yes. Let's take a look at some numbers.