Youtube and Netflix only slow in apps and on mobile

Hi,

I'm running Sophos UTM v9.705-3 on an APU4D4 behind my ISPs modem and observe partially issues with YouTube, Google Playstore and Netflix.

The following devices are involved (additionally I have several basic switches and unify access points running.):

1. TV
   1. Connected directly to my firewalls internal network port (100Mbit/s)
      1.  YouTube app is stuck at around 2Mbit/s. Higher than 720p is not possible without constant buffering issues
      2. Netflix speed test results in less than 50Mbit/s
   2. Connection directly to my ISPs modem (100Mbit/s)
      1. YouTube app is running every resolution without any buffering issues
      2. Netflix speed test shows always ore than 90Mbit/s
2. Mobile
   1. Connected to my internal network via wifi
      
      1. General speed test maxes out my ISPs limits
      2. Youtube app stuck at 2Mbit/s
      3. Playstore loading updates extreme slowly
   2. Connected to the mobile network
      1. General speed test matches my phone ISPs limits
      2. Youtube app running all resolutions without any buffering issues
3. Macbook
   1. Connected to my internal network via wifi
      
      1. No issues at all. Everything is running fine without buffering issues.
4. PC
   1. Connected internal via network (two switches in-between)
      1. No issues at all. Everything is running fine without buffering issues.

The following is temporarily DISABLED in my UTM:

• Intrusion prevention
• Web filtering
• Country blocking
• QoS

I monitored several logs while I was trying to run a 4k video on my phone. I couldn't find any relevant traces. The CPU of my APU is also not maxed at all. Ram looks ok, too. I have no idea right now, what the cause could be. Anyone any idea? Thanks for reading!

Kind regards, Andreas