You just have to know how to read phoronix: Michael Larabel is actually a linux hater and a strange diva with a knack for rancune and drama. It's a bit of a joke in the linux world. He's a nutcase, but subtract his editorial and neurotic comments from his site, and phoronix is a decent news aggregation page.
He recently admitted that he had been running linux in a VM in OSX on his Apple retina macbook pro, and that now that he has received another laptop, and has installed linux on bare metal, he wishes he had stayed with OSX. He just runs a linux site for the money, so don't read his articles, click on the links in his articles and go right to the source.
Now to the speed thingy: speed is relative.
Ubuntu Core is not slow, it's just not that fast either. There is a difference between "slow and fast" and "minimal and full featured". Ubuntu is slow, it's also a mess, Xubuntu is fast, it's also decent mainstream quality, both are based on exactly the same Ubuntu Core, which is pretty decent.
Some distros are remarkably fast for being so full-featured. Ubuntu core is pretty minimal by design, but only about as fast as a full blown RPM-distro, which has a lot more features, and as far as file system support for instance goes, the bleeding edge distros just have matured more, and the file system performance is quite a lot better, like sometimes almost 2 times better even.
The regular user that can see (not measure) the difference in speed in daily use between Xubuntu 14.04TC and Manjaro for instance, when both are configured with the same packages, must have an incredible sense of timing, because I can't see it. I see the difference in file system performance when I rsync an overlay file from SSD to HDD, arch does that a lot better, but the idea of rsyncing that overlay file is that you don't have to watch it, so ...
If you say that a distro is fast and another is slow, you have to know why you're saying that, and most people that talk about how fast their distro is, don't know why:
- Bleeding edge distros are usually faster, because a more modern kernel base, usually means better optimizations. Sometimes there can be performance regressions though, or the optimizations can maybe only affect certain hardware. For instance, the 3.13 kernel first showed some performance regressions because of the added features that haven't fully matured yet, but on AMD and Intel graphics systems, there is a considerable performance increase because of those functions, so someone running an nVidia graphics system might think a kernel 3.13 based distro is slower, whereas AMD and Intel graphics users might love the speed of the same kernel 3.13 based distro. The benefit of running bleeding edge, is not only the newer features, but also, bleeding edge distros gather experience faster with newer kernels and technologies, so they iron out regressions faster. A good example on how this impacts performance is the file system performance. Bleeding edge distros like Fedora, Arch or Gentoo have much higher file system performance than for instance Ubuntu Core. This is a considerable difference, although - as aforementioned - not always very visible to a regular user.
- Some distros just have a lot more features by default than others. RPM-distros have SELinux on by default. Yet they're not slower than for instance Ubuntu Core or Debian, that don't have this huge system implemented. Bleeding edge distros, because they're bleeding edge and need stability monitoring of the latest kernels, use daemons to monitor the kernel, so that in case of a bug, better data is available for diagnostics, this also costs performance, but they're optimized enough not to really feel the impact of that monitoring.
- A user can turn off features of a distro to enhance performance. Usually, this is only useful in particular use case scenarios. An example is the watchdog daemon, which is something most distros use. It's a hardware timer that writes settings every ten seconds by default to help a user reboot a system in it's current state if a critical event would happen. Most users could do without and not even ever notice it, and it does take a very tiny bit of system resources every ten seconds.
- The main reason why some distros are faster than others, has nothing to do with the distro itself, it has everything to do with the added software that is delivered by default, like the DE (still the biggest source of performance differentiation), the graphics drivers, proprietary software like flash, etc...