@Yapeng the slow upload when you change the file extension to .dummy indicates that the issue is nothing to do with the SD card.
My guess is that there is an incompatibility between WiFi module on the Duet and your WiFi router/access point. Here are some things you can try:
Try upgrading the WiFi module firmware to 2.1beta6 from https://github.com/Duet3D/WiFiSocketServerRTOS/releases/tag/2.1beta6 Try a different router/access point Check which channel your router/access point is using. You router WiFi configuration or status page may tell you, if it doesn't then use a WiFi scanner app on a smartphone. If it's operating on channel 1 then change it to a different channel, e.g. 7 or 11.