This design choice is a perfect example of "ZX Design philosophy"—prioritizing cost and cleverness over raw, brute-force power.
If you add a modern peripheral (like an SD card reader or Wi-Fi modem) to a Spectrum, you must respect this schedule. If you drive the bus during Φ2, you will clash with the ULA and corrupt the display. This design choice is a perfect example of
The ZX80 and ZX81 used discrete logic to generate video. The Spectrum needed color, but adding more chips would kill the budget. The solution was the —specifically the Ferranti ULA. This design choice is a perfect example of