This was my thought. Nanoseconds are an eternity. You want to be using Planck units for your worst-case analysis.
replyIf you go far beyond nanoseconds, energy becomes a limiting factor. You can only achieve ultra-fast processing if you dedicate vast amounts of matter to heat dissipation and energy generation. Think on a galactic scale: you cannot have even have molecular reaction speeds occurring at femtosecond or attosecond speeds constantly and everywhere without overheating everything.
replyMaybe. It's not clear whether these are fundamental limits or merely technological ones. Reversible (i.e. infinitely efficient) computing is theoretically possible.
replyIf you have a black hole as an infinite heat sink this helps a great deal.
replyNanoseconds is a natural unit for processors operating around a GHz, as it's roughly the time of a clock cycle.
replyIf a CPU takes 4 cycles to generate a UUID and the CPU runs at 4 GHz it churns out one every nanosecond.