Modern computers have multi-core CPU's running at 2-3 gHz, RAM space counting in the gigabytes. Modern graphics cards measure their insane parallelisation in the theraflops.
These modern systems can handle load easiliy. In the case of modern computers you can achieve greater speed under a greater load. With less load comes less speed.
But what if you're having an older system? Parallel would kill you. The background services, drivers, applications, the GUI all run on the smae thread (with time slicing) as your favorite game. This is slowing things down horribly. So the problem is not speed on these systems, it is just load.
This ammount of load to increase speed in devastating for older computers and they crumble onder it.
So Ubuntu is faster on modern systems where Arch is actually slower.
On older systems Arch is faster than Ubuntu.
Load != Speed