I understand that you did not change anything, but in vWLAN go to Configuration tab -> Wireless -> AP Templates. Click on the AP template that the APs are using. Scroll down and expand the channel list by clicking the button. Make sure that all of the channels under the 5GHz radio are in the left portion of the box. See picture for how it should look. Make the change and run the domain task if this is applicable to you and test.
I forgot to ask, what band are the devices connected to when they experience the issue, and what wireless NIC is being used at the client?
Also depending on your environment you might want to look at setting the 5 Ghz radio channel width to 40 Mhz to allow to VHT mode, some Intel WiFi cards come default to VHT mode and did not like the 20 MHz channel width but that very may have been a driver issue at the time. Also the 19xx series seem to need reboots more often in same cases, more so in noisy RF or busy environments.
Make sure you are following vWLAN Best Practice Recommended Settings. You also mention that when you first connected them, the speeds were good. This changed three days later, so that begs the question: How many clients were connected on the third day? Were the APs actually in production at that point?
Keep in mind that wireless is (at least currently and definitely for the 1920s) a half-duplex shared medium, and these 1920s are the oldest and lowest powered APs currently offered by ADTRAN. They are only 802.11n, a technology that has been surpassed by 11ac and soon-to-be 11ax. Basically, the 1920s are two generations behind. With all of this in mind, you may be running into standard wireless contention problems. How does arbitration work in an 802.11 WLAN? may be helpful.