This is a bug which is there back to mostly the beginning of the Insider Program (build 9xxx). I have reported it several times over the past 1,2 years over the insider tool, but also in the MSDN forums. Like you can see, it was ignored and got into final (RTM) version (nice work Microsoft).
The problem is simple, and you can monitor it yourself. Open task manager or CPUz, and you will notice, that Windows 10 never throttles down the Atom CPU to its lowest energy state (500-800MHZ), and just idles at around 1.3-1.8GHz. If you google a little bit about this, it seems it's a bug related to Intel Turbo Mode. I am not sure if it's an Atom (baytrail) only bug, or related to other CPUs too.
About the connected standby drain, yes, it is also true. You can simple monitor it yourself with the powercfg tool. Creat a bat file with "powercfg /batteryreport". It will create a html file with battery stats. Scroll down and youll see the last drained watt/energy over time period.
A normal, and also by Microsoft declined value for connected standby is about 0.3mW/h. Every mobile device which has connected standby certificate needs this to fulfill. And here comes the issue, that Windows 10 on the Venue 8 Pro drains about 1.1mW/h (for me), which is about 3 times over the max limit, leading to a 3 times faster drain. I am talking just about the standby drain here.
Tested this last week with 10240 (clean install) => bug present, went back to clean Windows 8.1, and got a normal drain of 0.35mW/h again. Also the throttling worked again.
I suggest not to upgrade to Windows 10, sadly, for your mobile devices, until Microsoft will finally notice this. Maybe it helps when you also open a forum post on the MSDN forums about this, and also post that other people have this issue too.