@lurker888
More tests have been ran, results are unfortunate - really no change at all in behavior. Same setup as before:
300/300 Mbps symmetric internet service, provided by Verizon FiOS, FTTH.
Testing client is 2020 MacBook Air M1 laptop.
E50UG was upgraded to 7.19.1 (package/routerboard) and reset to complete factory defaults.
Control Test - MacBook Air M1, hooked directly into provider ONT via dongle/ethernet cable.
Speedtest by Ookla
Server: Cox - Nova
ISP: Verizon Fios
Idle Latency: 8.77 ms (jitter: 0.19ms, low: 8.61ms, high: 8.84ms)
Download: 305.57 Mbps (data used: 316.9 MB)
109.22 ms (jitter: 28.78ms, low: 8.66ms, high: 162.99ms)
Upload: 341.07 Mbps (data used: 402.8 MB)
22.54 ms (jitter: 0.80ms, low: 9.04ms, high: 52.23ms)
Packet Loss: 0.0%
So, normal behavior (full CIR bandwidth) with the MacBook hooked up by itself, as expected.
Test #1 - MacBook Air M1, plugged into E50UG port 2, E50UG port 1 is plugged directly into provider ONT.
NOTE: Router left at factory defaults.
Speedtest by Ookla
Server: Cox - Nova
ISP: Verizon Fios
Idle Latency: 13.60 ms (jitter: 0.49ms, low: 13.08ms, high: 14.10ms)
Download: 310.19 Mbps (data used: 317.7 MB)
123.55 ms (jitter: 44.93ms, low: 8.83ms, high: 653.65ms)
Upload: 9.74 Mbps (data used: 15.7 MB)
11.73 ms (jitter: 23.11ms, low: 9.54ms, high: 625.87ms)
Packet Loss: 1.3%
Same results as before - upload speed between ~10-12 Mbps.
Test #2 - MacBook Air M1, plugged into E50UG port 2, E50UG port 1 is plugged directly into provider ONT.
NOTE: fq-codel queue created w/ defaults, and assigned to ether1 interface.
Speedtest by Ookla
Server: Cox - Nova
ISP: Verizon Fios
Idle Latency: 9.13 ms (jitter: 0.05ms, low: 9.07ms, high: 9.18ms)
Download: 306.81 Mbps (data used: 321.5 MB)
122.21 ms (jitter: 45.28ms, low: 13.10ms, high: 546.59ms)
Upload: 8.05 Mbps (data used: 4.2 MB)
10.67 ms (jitter: 5.91ms, low: 8.72ms, high: 360.89ms)
Packet Loss: 5.0%
Latency a little lower, maybe - but packet loss higher
Test #3 - MacBook Air M1, plugged into E50UG port 2, E50UG port 1 is plugged directly into provider ONT.
NOTE: fq-codel queue created w/ defaults, and assigned to ether1 interface.
NOTE: tx and rx flow control forced to ON for ether1
NOTE: Test ran five or six times per your request; below is the final run
Speedtest by Ookla
Server: Verizon
ISP: Verizon Fios
Idle Latency: 9.14 ms (jitter: 0.11ms, low: 8.92ms, high: 9.19ms)
Download: 309.25 Mbps (data used: 292.7 MB)
107.61 ms (jitter: 29.51ms, low: 8.74ms, high: 160.26ms)
Upload: 10.28 Mbps (data used: 17.3 MB)
6.07 ms (jitter: 15.43ms, low: 3.70ms, high: 353.99ms)
Packet Loss: 0.0%
No change at all, really.
Very depressing. I was hoping for at least some improvement 
EDIT: My results are basically identical to the person that started the reddit thread I linked originally here. He tried everything he could, eventually gave up and bought Ubiquiti device. Also notable some people claim sticking a dumb switch in front of E50UG magically makes problems go away. I do not have spare dumb switch unfortunately.
Just going to leave this here:
