In the MAX7221 datasheet on page 3, there is a spec called "data-to-segment delay." The maximum for this spec is 2.25ms. This is the time it takes from a completed SPI command to a change in the LED driver outputs. While the SPI clock can run up to 10MHz, the display cannot be updated that fast. 2.25ms translates to a display refresh rate of 444.44Hz. For reference, high end televisions update as fast as 240Hz, so 2.25ms is actually quite fast.