The Z80 generates a maximum of one request per microsecond. The CPC also requires two memory accesses per microsecond for reading video data.
The CPC specs 4164-20 DRAMs. These require 330nS for a read or write cycle. The CPC also uses the optimised sequential CAS cycles to read the two video data bytes. [ https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/the-cpc-revision-zero-article/msg243769/ Source]
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