SB6141 not giving full speed after service change?

I have an SB6141 that I’ve been using forever. It got 400mbps download when I had the 8/500mbps service (which I signed up for in ‘21.) and I knew that was the maximum of the modem, which I was fine with.

But I recently switched to 12/200 service instead and I’m only getting 100mbps download, tops.

i know the modem is old but is there a reason I’m getting so much less than maximum speed? (Is it something to do with the number of channels, etc.?)