Both of these replies or any others like it just aren't true. There is no explicit relationship between the number of features/functions and software efficiency/performance. The number of functions often correlates with how easy a software product is to use, but that's not what the OP was asking about. If anything, it's the kinds of functions that matter, not their number, but W10M doesn't do anything that WP8 didn't which would be a drain on efficiency/performance. The real cause is down to something else entirely...
The primary reason why each version of WP/WM has been less efficient than the one before it, is because with each iteration, the OS has become ever more similar to the desktop/server OS. Desktop and server OSes are designed and built with a completely different set of requirements in mind than are OSes which are designed specifically for small, resource constrained devices like smartphones.
WP7 was based on WinCE, which was a real-time OS built specifically for small, resource constrained embedded devices that had to run for years without requiring any maintenance or experiencing any down time. That's why WP7 was so efficient and robust. In contrast, W10M is based on the same code base that powers desktops and servers, which focuses more on being highly adaptable to a huge range of very different hardware platforms (WinCE only ran on a very specific set of predetermined platforms and there was no way to change that) and on making life easier for developers. WP8 is somewhere in between those two.
In terms of functions/features, W10M and WP7 aren't even that different. The big difference between the two OSes are in the apps that ship along with the OS (mail, groove, etc). That's where 95% of the differences in features are. We could port all those apps back to WP7 and even add to WP7 the few features W10M has which WP7 lacks, yet the WP7 device would still boot in 1/4th the time and be no less efficient than it was before that effort.
For most software companies, reducing development costs is more important than improving software performance. That's definitely true for W10M as well. That's where we're at now.