Xfinity throttling - not achieving paid throughput levels

I've read other posts on xfinity performance and haven't seen any clear answers from Xfinity.  I've been running performance testing and observing that xifinity network is inconsistent.  I'm running tests over a laptop hard wired directly to the 2.5G port on the xfinity router with a CAT6A cable.  I confirmed the network adapter is set to 1000/1000 and is capable of achieving speeds equal to my xfinity subscription of 1200 DL / 40 UL via CLI speedtest.

C:\speedtest>speedtest.exe

   Speedtest by Ookla

      Server: Comcast - Seattle, WA (id: 1782)
         ISP: Comcast Cable
Idle Latency:    13.46 ms   (jitter: 1.37ms, low: 12.51ms, high: 15.27ms)
Download:   939.34 Mbps (data used: 1.1 GB)
                 23.59 ms   (jitter: 4.09ms, low: 9.81ms, high: 76.16ms)
Upload:    41.29 Mbps (data used: 28.3 MB)
                  5.55 ms   (jitter: 2.15ms, low: 4.02ms, high: 59.63ms)
Packet Loss: Not available.

Next, I ran iperf3 tests to run interative tests over time.  The throughput on xfinity is very inconsistent and appears to be throttling.  The throughput starts out high (800+ mbps) but drops down to sub 100mbps with an overall average of 340mbps.  This is way shy of the 1000mbs (network interface limit) I would expect paying for 1200mbps DL.  I masked the server IP for privacy/security. 

C:\iperf3.17_64>iperf3 -c xxx.xxx.xxx.xxx -p 5000 -t 30 -R
Connecting to host xxx.xxx.xxx.xxx, port 5000
Reverse mode, remote host xxx.xxx.xxx.xxx is sending
[  5] local xxx.xxx.xxx.xxx port 57810 connected to xxx.xxx.xxx.xxx port 5000
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.02   sec  76.1 MBytes   629 Mbits/sec
[  5]   1.02-2.00   sec  79.4 MBytes   673 Mbits/sec
[  5]   2.00-3.01   sec   106 MBytes   885 Mbits/sec
[  5]   3.01-4.01   sec   105 MBytes   876 Mbits/sec
[  5]   4.01-5.02   sec   107 MBytes   896 Mbits/sec
[  5]   5.02-6.00   sec  77.9 MBytes   661 Mbits/sec
[  5]   6.00-7.01   sec  64.2 MBytes   537 Mbits/sec
[  5]   7.01-8.01   sec  67.6 MBytes   565 Mbits/sec
[  5]   8.01-9.00   sec  68.0 MBytes   577 Mbits/sec
[  5]   9.00-10.01  sec  70.6 MBytes   590 Mbits/sec
[  5]  10.01-11.01  sec  75.4 MBytes   630 Mbits/sec
[  5]  11.01-12.02  sec  63.4 MBytes   528 Mbits/sec
[  5]  12.02-13.00  sec  14.5 MBytes   123 Mbits/sec
[  5]  13.00-14.00  sec  10.2 MBytes  85.6 Mbits/sec
[  5]  14.00-15.01  sec  11.2 MBytes  93.9 Mbits/sec
[  5]  15.01-16.01  sec  10.5 MBytes  87.8 Mbits/sec
[  5]  16.01-17.00  sec  10.0 MBytes  84.8 Mbits/sec
[  5]  17.00-18.01  sec  11.5 MBytes  95.9 Mbits/sec
[  5]  18.01-19.01  sec  13.0 MBytes   109 Mbits/sec
[  5]  19.01-20.00  sec  13.0 MBytes   110 Mbits/sec
[  5]  20.00-21.01  sec  14.9 MBytes   124 Mbits/sec
[  5]  21.01-22.01  sec  15.4 MBytes   128 Mbits/sec
[  5]  22.01-23.02  sec  16.2 MBytes   136 Mbits/sec
[  5]  23.02-24.00  sec  17.9 MBytes   152 Mbits/sec
[  5]  24.00-25.01  sec  18.6 MBytes   156 Mbits/sec
[  5]  25.01-26.01  sec  19.6 MBytes   165 Mbits/sec
[  5]  26.01-27.01  sec  15.4 MBytes   129 Mbits/sec
[  5]  27.01-28.01  sec  14.5 MBytes   122 Mbits/sec
[  5]  28.01-29.01  sec  13.9 MBytes   116 Mbits/sec
[  5]  29.01-30.00  sec  13.6 MBytes   115 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-30.03  sec  1.19 GBytes   340 Mbits/sec  3226             sender
[  5]   0.00-30.00  sec  1.19 GBytes   340 Mbits/sec                  receiver
 iperf Done.

I can repeat this test, at different times of the day and the results are always the same.  There is a massive swing from high to low throughput, usually 75%-90% decrease in throughput.  

Using the same laptop, on a different (non-xfinity) network, running the same iperf performance test to the same iperf server, I get the following results:

C:\iperf3.17_64>iperf3 -c xxx.xxx.xxx.xxx -p 5000 -t 30 -R
Connecting to host xxx.xxx.xxx.xxx, port 5000
Reverse mode, remote host xxx.xxx.xxx.xxx is sending
[  5] local xxx.xxx.xxx.xxx port 64861 connected to xxx.xxx.xxx.xxx port 5000
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec   123 MBytes  1.03 Gbits/sec
[  5]   1.00-2.00   sec   140 MBytes  1.17 Gbits/sec
[  5]   2.00-3.01   sec   143 MBytes  1.19 Gbits/sec
[  5]   3.01-4.01   sec   141 MBytes  1.18 Gbits/sec
[  5]   4.01-5.00   sec   140 MBytes  1.19 Gbits/sec
[  5]   5.00-6.01   sec   144 MBytes  1.20 Gbits/sec
[  5]   6.01-7.01   sec   143 MBytes  1.19 Gbits/sec
[  5]   7.01-8.00   sec   112 MBytes   955 Mbits/sec
[  5]   8.00-9.01   sec   130 MBytes  1.08 Gbits/sec
[  5]   9.01-10.01  sec   132 MBytes  1.11 Gbits/sec
[  5]  10.01-11.01  sec   138 MBytes  1.16 Gbits/sec
[  5]  11.01-12.00  sec   129 MBytes  1.10 Gbits/sec
[  5]  12.00-13.00  sec   137 MBytes  1.14 Gbits/sec
[  5]  13.00-14.00  sec   137 MBytes  1.15 Gbits/sec
[  5]  14.00-15.00  sec   133 MBytes  1.12 Gbits/sec
[  5]  15.00-16.00  sec   139 MBytes  1.17 Gbits/sec
[  5]  16.00-17.00  sec   129 MBytes  1.08 Gbits/sec
[  5]  17.00-18.00  sec   142 MBytes  1.19 Gbits/sec
[  5]  18.00-19.00  sec   140 MBytes  1.18 Gbits/sec
[  5]  19.00-20.00  sec   139 MBytes  1.16 Gbits/sec
[  5]  20.00-21.00  sec   142 MBytes  1.19 Gbits/sec
[  5]  21.00-22.00  sec   142 MBytes  1.19 Gbits/sec
[  5]  22.00-23.00  sec   115 MBytes   967 Mbits/sec
[  5]  23.00-24.00  sec   122 MBytes  1.03 Gbits/sec
[  5]  24.00-25.00  sec   113 MBytes   946 Mbits/sec
[  5]  25.00-26.00  sec   124 MBytes  1.04 Gbits/sec
[  5]  26.00-27.00  sec   126 MBytes  1.06 Gbits/sec
[  5]  27.00-28.00  sec   124 MBytes  1.04 Gbits/sec
[  5]  28.00-29.00  sec   123 MBytes  1.03 Gbits/sec
[  5]  29.00-30.00  sec   120 MBytes  1.01 Gbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-30.01  sec  3.87 GBytes  1.11 Gbits/sec  3152             sender
[  5]   0.00-30.00  sec  3.87 GBytes  1.11 Gbits/sec                  receiver
 
iperf Done.

The change from high 1.2gbps to low 946mbps is minimal (20%) as compared on the xfinity network.  

Xfinity - Why am I not able to get consistent performance and throughput anywhere close to the level of throughput (1200mbps DL) that I am paying for?  I would expect executing performance testing tools, I would achieve throughput anywhere in the 800-1200 mbps consistently, not a range up to 90% off my subscription levels.  This explains when I'm working from home, all file transfers from work to home take exponentially longer than on other networks.  

Thanks