Unlike the ZX Spectrum or the Amiga, where bus arbitration is restricted to the "chip RAM", on the CPC it also applies to ROM access and to RAM expansions. So the Z80 always runs at the same speed, regardless of the type of memory being accessed.
Last but not least, bus arbitation also applies to I/O access.
Note: On Amstrad Plus, the ASIC also has to handle DMA instruction fetch from RAM.