I've just found this thread http://forums.channelregister.co.uk/forum/1/2011/11/02/fujitsu_k_super_10_petaflops/
in which Steve 48 (MIA since 2012) says "Reg units: I refuse to accept Petaflops - can we have it in terms of ZX81s instead?" and brainwrong (MIA for the last 18 months) responded:
Excellent idea! Slow mode or Fast mode?
The ZX81 didn't do double precision, it used a 40 bit format (documented in the excellent manual).
I don't know the flops rating of a ZX81, and I should think that the difference between adds and multiplies (done in software) would be much larger than modern hardware, which may complicate comparisons.
For a rough idea, a mandelbrot renderer I wrote in BASIC on a CPC464 (same Z80 running at similar speed, also 40 bit FP) achieved about 166 iterations per second. That was 4 adds and 4 multiplies, giving a whopping 1333 Flops!
Re-writing it in PASCAL tripled the speed, at the expense of reduced precision of 32 bit.
That was still so dreadfully slow that I re-wrote it again in Z80 assembler, bumping the precision back to 40 bit with my own routines. That ran at double the speed again, 1000 iterations/sec. Here I was able to replace a multiply by 2 with a single INC instruction, so I'll only claim 7 ops per iteration for 7 KFlops. I was still running renders up to 2 days at 320x400 resolution.
I have no idea how fast double precision could be done on a Z80, which is what is needed for a true comparison, maybe someone has done it and knows?
So, if I understand correctly (fat chance!), the ZX81 would need 4x10^12 hours, so that's roughly 4x10^9 years... so if not the heat death of the universe, then certainly getting a bit close to when the Sun will enter its Red Giant stage and engulf the Earth.
4 billion years per move... I'll never complain about playing a boring board game again!