Here is a TechTarget article that might be useful to you. This site has info for setting up RAID under Linux.
This article has some good information about RAID levels and other storage related terms and concepts.
Its real simple, say you have 100KB to write to disk but you can only write a 20KB chunk at a time and it takes 2 seconds to write each chunk!
So if you have a single disk it will need 5 * 2 seconds to write the data! ie 20 seconds
If you have five disks (spindles) you can write each 20KB chunk in parallel and now it only takes 2 seconds (plus a little bit, ie the time it takes to issue write, write, write, write & write commands) actually for RAID5 writes its a bit more complicated as all these seperate chunks need to be XORed together and then the resulting chunk written to a separate spindle.
So the more spindles you have the better! or so you would think, BUT (and there is always a but) you need to consider what happens if one of the disks fails (i mean, thats why you went to RAID5 in the first place isnt it?) then to reconstruct the missing data, all the remaining disks need to be read and the data XORed. This obviously slows everything down and the more spindles the longer the time. Although most RAID engines will allow RAIDsets of 30 or more spindles, most people would hesitate to recomend more than 15 at a maximum (although there are some RAID controllers such as th HP EVA which will allow Virtual RAID5sets of 200+ spindles (it actually has recovery units of 5 spindle chunks)