不要求 class 10：
The card class determines the sustained write speed for the card; a class 4 card will be able to write at 4MB/s, whereas a class 10 should be able to attain 10 MB/s. However, it should be noted that this does not mean a class 10 card will outperform a class 4 card for general usage, because often this write speed is achieved at the cost of read speed and increased seek times. (
https://www.raspberrypi.org/documentation/installation/sd-cards.md)
No matter what, you will not much exceed 20 MB/s (read or write) on the pi's SD card bus. Have a look at the chart here, and notice the very fastest write speeds are 21 or 22 MB/s. I've done my own test of class 4 vs. class 10 -- notice they had identical read speeds of ~20 MB/s indicating both cards were limited by the pi's SD card bus. (
https://raspberrypi.stackexchange.com/questions/12191/what-speed-of-sd-card-is-better)