The problem here is you're all still talking in generalities - and
asking for an optimized solution. Some more specifics would help.

Give at least SOME actual examples of the operations you are performing
- not abstracts.

I think everyone would agree that the single biggest speed factor is
going to be disk access. Here again you need to get specific about your
operations. If your operations are "write bursty", then investing in
memory/caching controllers may bring significant results relatively
inexpensively. If you are actually performing constant disk writes
without any significant relief - then you need to spend some serious
money on your storage system. If the disk writing operation is
continuous - you'll run out of cache memory regardless of how big you
grow it or how many processors you throw at it.

Daniel