This joke is not just applicable to the ZX81, in fact the general idea of a RAM shared as video RAM and CPU RAM lives on today, just much less noticeable. And as an older example, the Commodore Amiga performed much worse CPU-wise if you used highres, you had "chip mem" and "fast mem" as a consequence etc.
The ZX80 and ZX81 just took a somewhat, ahem, less subtle (but efficient!) approach to get those cycles to the CPU :)
If you are over 47 and from the UK then this joke is extremely funny. Laugh out loud funny. I don't believe anyone outside this niche truly gets it. Brilliant.
FAST mode disabled the screen raster output to allow more cycles for user code. The TV monitor was free to improvise its output, which for me was alarming rolling garbage and an audible whine.
Turbo mode changed the clock speed, which was a less hectic experience.
As robotresearcher says in a parallel comment chain, FAST mode was without the screen running. Normally the ZX81 only did anything during the vertical blanking, most of the time it was drawing the screen. If you enabled FAST mode then it could go ten times quicker or more.
Sure the ZX81 sold internationally but it was so much more successful in the home market than overseas.
It may have sold in some numbers in North America, but remember that Commodore had a trade-in offer and some chains like Fast Eddie's offered the T/S 1000 at a bargain price solely to allow people to get the discount. Commodore reportedly used some of them as doorstops!
