Multi-processing is only exploited when you design your system to exploit it. There was no wide-spread auto-parallelization of algorithms on these systems, at least not in the 80s. Certain database operations could be parallelized if you wanted to pay for it. The support on the application end was scant IIRC.
Decimal operations on modern ia32, ia64 and arm are implemented using 64-bit integers and not slow at all.