About Nintendo Switch SD card reader speeds:
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In order to improve your gameplay experience, using a microSD card with a higher transfer speed is recommended.

High-speed microSD card guidelines:

UHS-I (Ultra High Speed Phase I) compatible
 
Transfer speed 60 - 95 MB/sec (the higher the transfer speed, the better gameplay experience on Nintendo Switch)

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May 8th, 2018 @ 12:18
https://www.nintendo.com.au/help/switch-storage-and-data-management