CM3000 not receiving full speed after upgrading to 2gb tier

I had similar problems.  Toasted Arris modem, then Motorola replacement showed signs of thermal stress.  Xfinity blamed modem, so I bought a new CM3000, only to be told the new CM3000 was broken right out of the box.  Tier 1 and 2 said they saw no problem.  However, that's because Xfinity uses a far looser spec tolerance.  The CM3000 downstream was built for +/- 7 dBmV channels.  Xfinity says all is fine up to 10 dBvM.  Truth is that a bunch of channels above 7 dBmV is not only generating too much heat, distortion creeps in.  But loosening the spec is a lot easier than replacing old line amps with large slopes.  Upstream values shouldn't be much over 44 dBmV, and certainly no higher than 48 dBmV.  However, Xfinity will approve upstream power in the 50s.  Now, it may seem like a few dB isn't much, but a 3.5 or so dB gain equals twice the power.

If your upstream is too hot, Xfinity probably won't correct it and blame it on your equipment.  That leaves you with no option than to remove splitters that attenuate the signals by 3.5 dB each.  The attenuation brings down the upstream signal, so your modem is commanded to higher outputs.  It gets to loud fighting the splitter attenuation.  Removing splitters also boosts the downstream levels.  If boosted too much, you need to apply a forward attenuator.  However, it may be hard to find one if you're scheme is mid- or high-split.  Most work on the old low split, where the up/down guard band is 42 to 54 MHz.

Next, tier 1 tech support only understands redlines on what the modem reports.  What the modem reports is an average power level across a bandwidth.  QAM256 bandwidths are generally small, like 6 MHz.  But once you get to the OFDM channels, the bandwidths are generally 96 MHz.  Not only is that larger and more sensitive, it generally overlaps major cell phone frequencies.  How does cell phone RF inject through shielded cable.  Xfinity will tell you that your 20 feet of cable from the demarc is the culprit.  In actuality, the Xfinity feed may be miles of very old cable and equipment that's ripe for RF injection.  The best way to deal with this is to get a RTL-SDR spectrum analyzer for $50 and map out the OFDM frequencies by attaching it to your cable feed.  If you see narrow spikes sticking out above signal and related 8 to 10 dB "divots", that's cell RFI on the Xfinity feed.  If you see slopes on the OFDM signal, or rolling hills, a sign of more trouble.  

Generally, if there are OFDM dropouts, this shows up as errors.  To an Xfinity tech living at OSI layer 2, there's nothing wrong with millions of corrected errors per minute because they're all corrected.  But to your equipment, correcting a lot of errors and dealing with the asynchronicity of messages at layer 4 can slow things down.  The sender does not receive required ACKs as expected, so the sender slows down his bitstream assuming the link is congested. 

Xfinity left me in the lurch with this, as they don't want to spend the money to fix a system that partially works.  I've found some things I can do that help.  My feed is essentially DOCSIS 3.0 with a kluged OFDM carrier, which is fully capable of Gbps speeds if Xfinity fixed their cabling (or implemented real DOCSIS 3.1 amps that actively tweak the OFDM waveform).  First is remove all splitters and splices, especially old ones, and see if up/down power levels can be maintained (44 dBmV up, 7 dBmV down).  Your upstream modem report will tell you whether you have low/mid/high split.  Most forward attenuators work only on low splits.  If a splitter or splice (barrel connector, not soldered wires) is necessary, make sure it's rated to 3 GHz.  Terminate all unused ports with a 75 ohm cap.  Either remove open/unused cables or cap both ends with a 75 ohm terminator.

Next, disable anything on your computer that might further delay signal processing.  An AI like Claude can be really helpful tracking things down.  Do not allow power saving on network cards.  Unless you have a need for specialty equipment needing IPv6, kill IPv6 on the network card and let a powerful (i.e., not Xfinity gateway) router handle the translations.  There's a number of advanced network card settings in device manager that can be adjusted, like increasing receive/transmit buffers.  If you don't have such controls, Claude can direct you to the registry locations.  Of course, write down initial values and changed values for breadcrumbs.  When you do speed tests, Task Manager - Performance can be set to view the use of CPU cores.  There are ways to spread the load if a CPU core becomes overloaded (>90%).

Is it necessary to plug your computer directly into the modem to do a speed test?  Of course NOT!  It's a gaslighting device Xfinity uses.  If you have a couple home computers and you turn off cell phones and RF appliances (including printers), an idling computer is not going to use much bit rate that substantially interferes.  It may carve 50 or 100 Mbps out, peak, but not consistently.  If you have a question, just open the Task Manager - Performance on the "idle" computer, select "Ethernet" or wi-fi, and it will show you exactly how much bitrate the idling computer is using.

If you have questions, try Claude and describe your setup.