Linux software-RAID4/RAID5 and CPU usage

As TomTom has said, it's not as brutal as it used to be; but then disc drives have got bigger while CPUs were getting faster.

Which is why it's not a good idea to do RAID-5 in software unless you really don't care about performance. RAID-5 in hardware at least ensures there's a reserved processor whose sole job is to do those parity calculations; also, the hardware will often have things like NVRAM to prevent array corruption, and the ability to optimise the calculations, eg by knowing that a whole stripe is being written and skipping the (hugely expensive) read-modify-write cycle in favour of a simple parity recalculation.

Even with hardware RAID acceleration, applications that modify very small chunks of data at a time - particularly databases - can perform very badly indeed on RAID-5 (and RAID-6, which is even more expensive in terms of parity calculation). For that kind of application, just put your hand in your pocket, get the extra discs, and do RAID-1+0.

You mean for a modern CPU with like 6-8 cores all many times more powerfull than that on a RAID card?

Not today, This is 2013. CPU's can handle a LOT of stuff these days. You will have problems using up the power of a single core, unless you run quite a lot of SSD's to RAID.