oh i thought the speed was just picked because it's still within spec of the CPU, not because you already considered something faster but couldn't do it due to hardware limitations.
hmm, sorry on the spot i can't help much, the video circuit alone is already way more complex than everything i've made so far (though i only ever had 128 Macrocells to play with)
the fastest 5V SRAM i could find is this 10ns 512kB one:
https://www.mouser.com/ProductDetail/87 ... 28AL-10KLIit's pretty cheap right now, though the price is slightly higher than when i bought a bunch of them like half a year ago.
If i'm reading the documentation correctly, accessing VRAM is done via a small memory window through the CPLD.
if your Video Circuit always fetches data at 25MHz (and in the opposite half-cycle to the CPU), then bumping the CPU up to 25MHz shouldn't cause any issues when accessing VRAM at the same time.
technically that would also mean you could throw away the whole "trhough the CPLD" thing and have the CPLD and CPU share the same data bus to the VRAM chip via a simple buffer between the CPU bus and CPLD bus.
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but that wouldn't work that well since the CPLD is also responsible for loading the boot code into the CPU's RAM. (unless you use 74x245's for both the data bus and the address bus so the CPLD can take over the CPU bus for a short amount of time through the buffers)
on a side note i'm a bit confused by your comment "parallel ROMs are getting harder to come by", Parallel FLASH chips are the modern versions of EEPROMs, they are still being made and are pretty cheap. the SST39SF0x0 series of FLASH chips are a great example. though the speed is still lacking, with an access time of 70ns my 20MHz 65C02 needs 1 wait state to properly access the chip.
but i can still understand the reasoning for using an SPI flash, it's more compact on the PCB and allows the boot code to run in RAM with no wait states or extra address decoding.
overall it's a very nice looking project you got and i wish you luck with any overhauls of the system that you might do (if any)