Microsoft made an interesting off-stage admission at the Build 2014 - Windows Phone is an operating system for low-end hardware at heart. Developers working on the platform are required to develop on the most limited hardware available, and make sure Windows Phone works perfectly on it, before bringing it to faster phones. Fast chips and big screens are good fun, but they are merely testing tools.
Microsoft explains that this approach makes it easier to bring quality performance to all hardware ranges, instead of the other way around - building the OS without being conservative, and down-scaling it to fit weaker devices later. This “build for low, optimize for high mentality” could explain why the cheap Lumia 520 is such a capable little performer considering its modest specs.
Meanwhile, its Windows Phone 8.1-running successor, the Lumia 630, is expected to be just as zippy, without having the advantage of significant hardware upgrades – its chip is faster, but it's coupled with the same 512MB RAM, for example. Moreover, this approach will let Microsoft bring a well-optimized Windows Phone 8.1 across all WP8-based Lumia smartphones, which is very fair to the customer and helps avoid fragmentation.
Of course, it's not only sensible development that helps Windows Phone behave equally for everyone. Not unlike Apple, Microsoft restricts its hardware partners to very specific hardware configurations, and optimizes its operating system for them. It's easier to get the best possible performance out of devices if you know what to expect from them.