The reading speed of SD card is too slow, only 1.4mb/s. How to improve the reading speed of SD card

What is the release version you are using? I remember max-clk-limit has already been removed from Nano dts for quite long.

The device tree is also wrong. Lacking of cd-gpio and vmmc-supply. That makes you not able to remove the max-clk-limit.

For old release <=32.2. Below thread includes the need.

And this thread is for rel-32.5.1 that was just done last week.