My impression (and I haven’t done any back-to-back testing for over five years) is that if you put two Linux systems in the same configurations, the speed is essentially the same. That said, the defaults may be wildly different and one system may offer options that the other either doesn’t offer at all, or doesn’t easily offer.
While services are the thing that most people comment on, I think that set up for the disk partitions is something that is often overlooked. Firstly what are the respective partition types (probably ext3 or ext4, but reiser3 was fast, back in the day for general purpose usage, and not many people have been adventurous enough to go with reiser4)? Also the journalling options (assuming that you are using a journalling filesystem); Suse tends to be fairly conservative (low risk of data loss, rather than prioritising absolute speed).
Note also that side-by-side comparisons are difficult as partitions early in the disk will be faster than partitions late in the disk. Also, the partition layout (separate partition for home, ‘distance’ (stroke length) to swap, lots of partitions vs few partitions and how they are organised) makes a difference. So, you really can’t do side-by-side benchmarking, you have to do sequential installations, if you want valid timings.
Peculiarly, reducing swappiness is a popular ‘speed up tweak’ with Ubuntu users, but isn’t really popular anywhere else. (And, anyway, it is probably best described as increasing ‘responsiveness’ rather than ‘speed’, even when it works, and there is a suggestion that in some use cases it would be the opposite of constructive.)
And then there is GUI. How much of an impact this makes depends on how much RAM you have. The heavier weight GUIs use more ram (doh!), but if you have lots, that doesn’t really make much of a difference, but if you don’t have enough, then it could be a massive factor (…but then, on the other hand, using apps/applets from one GUI under another implies that you load the libs from both GUIs and that could be a ‘worst of both worlds’ situation, if you don’t have enough ram for that…).
As an example, which isn’t totally relevant, my preference is for KDE, but I’m finding that after a while it slows down, as it gets deeper into swap, so I’m currently trying out Gnome (which I don’t really like, but this is an experiment). I haven’t tried this out for long enough, but I’m not (yet??) seeing the slow down that I see with kde as its memory usage gradually rises and I’m not seeing the climb in CPU temperature. This seems potentially down to kde4 (or the apps?) leaking memory, and I wouldn’t really advise using KDE4.x for perf critical stuff, and there are probably big differences between different kde 4.x versions, so, unless the two distros use exactly the same kde 4 versions, it would be hard to compare.