network slow: 3 switches between router and pc

Hi,

 

i have a network that distributes an internet connection (1gbit) to 17 house staircases over copper with 6 endpoints each, so that at each endpoint a maximum of 32MBit up/down can be used and all subnodes are protected via VLANs, so that endpoints cannot see, disturb or connect each other.

Since a few weeks more endpoints were connected and speed problems started.
speed drops under 1MBit or fully stops, while the internet connection is most unused.

Since the endpoints are spread over almost 250m, a GS110tp switch was connected every 80m to get over the 100m limit.

All computers connected to GS108E switches behind these two GS110tp switches often have problems.

All switches have VLANs configured - configuration shown in the picture.
All stairway switches have 32MBit up/down limits configured, except at the uplink port.

I hope the structure is visible in the picture. Computers at the switches in the red rectangle have problems.

Where and how could I look for the problem? What could go wrong here?

 

i updated all firmwares yesterday, however no improvement.

 

the 90 days warranty is over, so I don't have free support from Netgear anymore. :(

Best regards,
Daniel

the network with some descriptions