RAID-4 and RAID-5 offer fault tolerance against a drive failure. This fault tolerance derives from the use of parity data, which must be computed when disk writes are done. This parity calculation is performed by the CPU and so if the CPU is heavily loaded, things slow down for writes (parity is not computed for reads by SoftRAID).
SoftRAID will use one CPU core for every drive that is part of the RAID-4/RAID-5, hence a dual-core machine (or even 4 core) might not run as fast even if idle, and under load will definitely see degradation (for writes).
The important point is that most workloads neither utilize all CPUs nor does the workload involve a lot of writing. But some workloads might, so a RAID-0 stripe may be more appropriate (e.g., processing a video stream to a new format). Still, few workloads will write a gigabyte a second, so specifics are what matter.
RAID-0 striping has very low CPU overhead (no parity to compute) and thus is all but unaffected by a heavy CPU load.