The Z80 was introduced, as I understand it, at a clock speed of 2 MHz. Later, Z80-based computers at 3.5 MHz started showing up.
In what year did this faster clock speed become available? (In production quantity, not just samples.)
The Zilog Z80 (2.5 MHz) was introduced in 1976.
In 1977, the Z80A (4 MHz) was available for $65 for the ceramic package and $59 for the plastic version.
The Z80B (6 MHz) was available in August, 1980 (p. 18) if not earlier.
The Z80H (8 MHz) was introduced in October, 1982 (p. 1).