Speed of Card Readers Speeds of Macs
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 480Mbit/s. Mac desktops use the PCIe bus to communicate with the SD card slot. Desktops can transfer data at a faster rate.
  
  Check the packaging that came with your SD media to determine the maximum transfer rate that your specific card uses.
  
  To determine the maximum transfer speed of your Mac, you can use the System Profiler. Go to the Apple () menu > About This Mac and then click System Report.
  
  If you use a Mac notebook:
  
  
  Select Hardware, then select USB.
  Select Internal Memory Card Reader and look for the Speed entry.
  
  
  If you use a Mac desktop computer:
  
  
  Select Hardware, then select Card Reader. 
  Look for the Link Speed entry. Computers that use the PCIe bus express their speed as GT/s.
  


About the SD and SDXC card slot on your Mac