Having used a 6121 and then ditching it for a 5341J due to the firmware/T3 issues, I have had no problems with the Zoom and have been very happy with it. As a disclaimer, I'm not faulting the modem in this discussion nor my wiring hardware, signals or CC (All are good btw; signals are nominal and wiring is new). I just think it's a difference in how each modem processes data, sort of like Intel vs AMD. The Surfboard uses a TI chipset, I believe, and the Zoom uses a Broadcom. At any rate...
(*The two charts above are BitMeter charts taken during a download; one is the Surfboard and the other is the Zoom just so you have a visual reference while reading this. The downloads I started are on the right of the chart; the yellow/red portion)
One thing I have noticed though throughout the months of using the Zoom is that for some reason, it seems to take time to get up to full provisioned/tiered rate speed whereas my Surfboard was immediately pulling data at full speed when I requested it. And the second effect could be just my mind, but the BitMeter charts above seem to bare this out, is that there is a somewhat 'dirtier' download connection, almost like noise on a carrier wave if one was looking at an oscilloscope. It seems to fluctuate speeds more variably during the download. The Surfboard always seemed to pop at full speed (w/ PowerBoost naturally) then cleanly transition to the real tier speed and remained until it closed the connection after it finished.
You probably won't even notice these issues unless you had a live bandwidth meter because almost every download "gauge" on the internet takes averages of the incoming data, it doesn't show real time flow, thus, evening out the peaks/valleys or other aberrations. The "ramp up" issue is the only thing I can really detect without a meter. I know it's a horrible testing medium to use but with YouTube when I had the Surfboard (Past its "Primetime" usage slot so I could get decent speeds), it always seemed that videos would immediately begin buffering without stopping and buffer well past what was being played even at the start of the video so it never stuttered or froze (even if selecting 1080p).
With the Zoom, it's like an old crumudgeony engine at times that once you yank the flywheel cable you have to kick it a view times before it runs full bore. It mostly seems to choke at the beginning leading to that inevitable buffer ring at 1080p and sometimes even 720p videos. And with speedtest or xfinity speed tests or any other flash or browser speed test, it always seems that the Zoom has to "accelerate" much like a car to get up to that full provisioned speed when I never had that issue with the Surfboard. And yes, I have tested those modems on off-peak hours and various times of the day.
In any case, I know this will probably fall on deaf ears (or eyes?) and there's probably not much to be done about it anyway but was just wondering if anyone else has gotten this issue with their Zoom modems.