SG2008 cannot reliably stream TV or games with VLAN port profile

My topology: Arris SB8200 --> Firewalla router --> SG2008P --> SG2008

• I have 1 Gbps from my ISP
• The SG2008 (v4.20.9) is powered by POE from the SG2008P (v3.20.10)
• All connections are Cat 6 cables (tested, known-good)

I have an Apple TV, an Xbox Series X, and a PS 5 connected to the SG2008. All of these work as expected when their ports are assigned the default "ALL" port profile. Only 1 of these devices run at a time.

The problem: When I assign a port profile isolating these to VLAN 20 (my IoT VLAN), all of them experience buffering pauses, and the gaming consoles experience jitter. CPU usage on the SG2008 is never higher than 11%.

Is my SG2008 defective? Or is it underpowered to handle streaming or gaming?