SF area: very slow network performance during business days (especially downstream)

Since signing onto Comcast for Business I've continued to have really poor downstream network performance during business days. Typically tethering to a T-mobile phone will provide better performance.

Typical day like today:

Downstream: 1.5-3.5Mbps (abysmal)

Upstream: 7-8Mbps (fine)

Ping latency: 20ms (fine)

Prior to Comcast for Business I had standard residential Comcast service with Boost (supposedly up to 150Mbps). The same issues occurred.

I've verified this through an ethernet connection to the modem as well.

At other times, the connection improves, but it's actually during the business day for when this service is most critical.

The best experience I had was when the technician installed the Comcast for Business modem with 20Mbps downstream (in theory it should actually be twice that).

What is recommended to resolve this?

Thanks!