New ISP, bad site to site performance. What data should I be collecting?

SMB here, we have three office locations. Two of our satellite offices (Sacramento and San Jose) switched to Comcast Biz Gigabit service early summer 2020, while our main site Livermore remained on our original ISP (TPX 50/50mbit) while Comcast did construction to bring two, one gigabit, circuits into our building.

Construction completed a few weeks ago and both internet and phone service were recently brought online. We made the switch from TPX to Comcast in the Livermore HQ this week. We immediately discovered that our site to site performance completely tanked.

I've been doing my best to eliminate any possible hadddrdware issue on our side, so all of the following IPERF3 tests were done directly connected to the Comcast gateways (not behind any firewall). I also added one other location/ISP (Sonic 1Gb symmetrical) to illustrate that I think this issue is within the Comcast network.

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Sender	Receiver	Bitrate/sec
SJ Comcast	Sonic	40.7 Mbit
Sonic	SJ Comcast	400 Mbit
Sac Comcast	Sonic	38.7 Mbit
Sonic	Sac Comcast	356 Mbit
Liv Comcast 1	Sonic	33.9 Mbit
Sonic	Liv Comcast 1	98 Mbit
Liv Comcast 2	Sonic	40.8 Mbit
Sonic	Liv Comcast 2	87.4 Mbit
Sac Comcast	SJ Comcast	9.36 Mbit
SJ Comcast	Sac Comcast	8.8 Mbit
Sac Comcast	Liv Comcast 1	913 Kbit
Liv Comcast 1	Sac Comcast	1.68 Mbit
Sac Comcast	Liv Comcast 2	620 Kbit
Liv Comcast 2	Sac Comcast	1.78 Mbit
SJ Comcast	Liv Comcast 1	772 Kbit
Liv Comcast 1	SJ Comcast	2.09 Mbit
SJ Comcast	Liv Comcast 2	702 Kbit
Liv Comcast 2	SJ Comcast	1.78 Mbit

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I understand that this Comcast product isn't an enterprise-level offering, but what the heck, we're talking less than 1Mbit when coming from our remote sites into either of our new HQ gigabit lines. It's clear that there are no hardware issues with my systems, when connecting to the Sonic ISP I'm getting "reasonable" (or at least livable) transfer rates.

Are there other tests that I should be running to help equip myself for the uphill battle with Comcast that I'm about to undertake?

Thanks!