Slow downloads on smartphones connected over AP

Hi,

we have a WiFi running for our smartphones mainly to update Android and apps. WiFi is offered through an AP55. Only smartphones can connect based on their MAC address. There is an own firewall rule for these connections with the following options active: HTTP scanning, block Google QUIC, detect zero day malware with Sandstorm, Scan FTP.

Unfortunately downloads are very slow. App updates take a long time and Android updates are canceled at a certain point by the smartphone itself.

As all updates are done over secure socket layer protocol and HTTPS scanning is not active I wonder what could be the reason. I checked the IP addresses that are used during update and always got to https://r3---sn-h0jeened.gvt1.com/ and https://r4---sn-h0jeened.gvt1.com/ so I excluded gvt1.com from HTTPS scanning, malware scanning and sandstorm. But also this showed no improvement on download speed.

Currently I wonder if the throughput of the AP55 is that slow?

 

Does anybody have any suggestions? Thanks.