Issue with Slow Sandisk Extreme 1Tb with MBPro mid 2014
Solution 1:
Maximum transfer rates are going to be slow compared to what the card can theoretically achieve. Macbooks with card slots run at USB 2 speeds.
From the Apple KB - About the SD and SDXC card slot on your Mac
How fast can my Mac read or write to an SD card in the SD card slot?
Mac notebooks use the USB bus to communicate with the SD card slot. They have a maximum speed of up to 480 Mbit/s. Mac desktops use the PCIe bus to communicate with the SD card slot. Desktops can transfer data at a faster rate.