I don’t think I see the same issue - even with a Mixergy and the same pump and PHE as you from one of my hot water runs (looked for one without defrosting to make it easier):
Could it be you’re just running your secondary Mixergy PHE pump too fast? Have you tried “2” speed rather than “3” to observe the difference? (And yes I know Mixergy just say put it on “3”!)
No, I think this is a Vaillant thing with their DHW modes
Now I know what i’m looking for (instantaneous COP), I need to do some playing with settings on the Vaillant Mixy side to work out what’s going on.
Time to hold my hands up - I WAS WRONG - the Mixergy can get good COP.
I did an extreme test today.
I fully emptied my 250L Mixergy It was 9C at the bottom and 12C at the top.
Settings used
When the run started it was 5.3C outside and finished at 2C outside.
COP of 3.59 across the whole run at those outside temps!!
If I cut the graph at 45C return, it was 4.04 COP!!!
So bascially, my wrong doing has been trying to blast full heat pump output to heat the water.
Adam Heat Geek calls this “heat source saturation point”, which I need to learn about on their Mastery course.
This is why most of the top of scops heat pumps on the heatpumpmon list use some form of Eco or reduced compressor mode for their hot water runs.
The obvious downside to doing Eco is that hot water runs can take longer.
This heating of 250L from 9C to 51C in Eco took 4hrs 10 mins.
Fine in the summer, not so good in winter when you want the space heating on.
As i’ve said before, a 250L tank with a 5kW heat pump is a tough combination.
My 5kW model on this run output 3.8kW down to 2.8kW because of the colder weather.
Spec sheet says highest output you’d see in Eco is about 4.5kW when 20C outside.
Obviously, on a 7kW, 10kW and 12kW model the Eco output would be higher.
Adam would get a max of around 5.9kW at 20C outside in Eco and 4.5kW at 2C outside.
Granted, emptying the tank was not something that happens every day.
My full blast DHW runs usually take about an hour. On Eco they would take two.
So there is a choice.
Shorter runs, full output, worse COP
Longer runs, reduced output, better COP
I’m gonna do a blog post all about this I think.
But the bottom line is that the Mixergy CAN get good COP, which on initial testing looks the same as coil based cylinders (more testing / comparing required).
I’ve been guilty of comparing apples to oranges.
My Mixergy at full blast, heating to higher temperatures (51C) versus coil based clyinder running Eco and heating to 40 to 45C.
Hope you find this update useful.
Awesome!
And as messaged, the newer revisions like yours also don’t disturb the thermocline allowing full access to the existing hot water at the top of the tank whilst doing the re-heat. (My older revision Mixergy from 2018 mixes the entire tank contents 
on a heat pump based recharge meaning I can only really recharge overnight)
Really interesting @Zarch. Is there any chance you can do a heat up from cold going at max? It be really interested to see how it compares? My huntch would be that it’s the starting from cold that’s the most important factor and the rate secondary… I may well be wrong about that but would be really interesting to see what the difference is…
What’s the primary
Figures in the COP maps would agree. There’s a difference to eco vs speed but that’s not the whole story
Here is an interesting observation from the COP maps (and me re-formatting them in this table).
Top is 5kW Eco (50 rps), middle is 5kW Normal (110 rps), bottom is the difference between the two.
So this tells us you get better COP across the board using Eco up to about 50C flow.
Once you get past that, Normal looks to yield better COP.
So this suggests that if you have a high target DHW temp, you might be better running in ‘Balance’?
Balance is ‘Eco’ until 45C flow and then ‘Normal’ after that.
But there is more to this than just COP on a sheet. Eco (and low compressor modes) work differently (low/slow) to achieve expected COP. It looks like Normal doesn’t get the expected COP listed here on DHW runs. Blasting full load at DHW doesn’t seem to allow it.
Have you tried a full reheat from cold on normal? When your reheating from cold there’s no thermocline to mix up in the tank. I guess outside temps are not conducive to such efficient heat ups now, so maybe a test for a warmer day?
Looks like they are introducing some new settings in May…
From Heat Geek’s video: Hot Water Settings Energy Suppliers Don’t Want You to Know!
Read more here: Mixergy and Heat Geek Integration • Mixergy
interesting Tim. Will take a look at that.
The Mixergy app have allowed less than 50 target for a while now actually without having to hack it via the API
I think they heard my moaning too. 
A lot of that Mixergy already supports, they may have a changed a couple of things though. Looks like they are potentially updating app to put these setting in one place.
I personally don’t use any of the automatic scheduling stuff, as I heat Mixergy overnight on OI tarriff, but for those using the automatic stuff, it looks like this might be new:
As for the resistor switch, fromy my experience/understanding a “pass-through sensor” wouldn’t change the behaviour with a Vaillant Arotherm+, it may do with other heat pumps though.
