Yes, I had the TS1000, a slightly modified version for the US market (with NTSC video output). I remember its built-in BASIC actually had "FAST" and "SLOW" commands. I love that they felt "SLOW" was an appropriate name for a command.
I guess I had always assumed this was about memory bandwidth, but this Alto article raises the possibility that the CPU may have been doing double duty (probably for cost reasons and not flexibility).
The way this actually worked is a giant hack, explained here: http://zx81.us/zx81vid.txt But basically yes it really was about 4 times faster in FAST mode.
I guess I had always assumed this was about memory bandwidth, but this Alto article raises the possibility that the CPU may have been doing double duty (probably for cost reasons and not flexibility).
And this documentation says it was "four times as fast", which is probably too much of a difference for just memory bandwidth: http://www.worldofspectrum.org/ZX81BasicProgramming/chap13.h...
I wonder if the implementation is as simple as just swapping the priorities of user code and display code.