Why is the OTA upgrade process slow?
The OTA upgrade speed is influenced by several factors, one of which is network interference. If you are operating in a noisy environment, it may help to address those issues first.
If you are upgrading several devices in parallel, the total bandwidth available for over-the-air transmission will be split proportionally between the devices. Although it may seem that the upgrade is going slower than possible for each individual device, the process will take the same amount of time as each device being upgraded sequentially.
If you are seeking a faster method to upgrade a large group of devices, review the Firmware Upgrades page and investigate the possibility of serial firmware upgrades.