The “sdcard reader” is just a socket, that is wired directly into the SoC. The performance of it is therefore DIRECTLY a function of the SoC’s implementation of sdcard, and also the software implementation/configuration.
According to rk3399 specifications, it complies with SD3.0 (ref: http://rockchip.wikidot.com/rk3399), which should support a maximum bus speed of 104 MB/s. And now of course you are scratching your head wondering why you never saw anything much over 20. Maybe take a look here: SD card SDR104 mode support