If you do the math, at 40 MHz, one clock cycle is 25ns.

If the motherboard only requires 20ns chips at 40MHz then putting 15ns chips don't make it any faster, because the cache is used at 40 MHz.
15ns chips will just consume more power than 20ns chips as they are capable of being faster even if capability is not needed.

If you put in too slow chips, the motherboard won't know this and cannot slow down. It will still use the chips at 40MHz and it won't work.

Having said that, even if 40 MHz is 25ns, there might be some other requirements why 25ns chips are too slow and thus some margin is required. I am not sure. But definitely anything over 25ns will not work on a 40MHz motherboard.

Thanks for the info. So 20ns would be fast enough. I have another 386 motherboard with Cx486DLC-33MHz that has 25ns cache chips - I guess they are fast enough as 33MHz clock cycle is 30ns.

About the clock cycles. I guess it is just the memory speed that is important and not the CPU frequency. As the 486 motherboards have 15ns caches.

I had a 486SX-25 motherboard with 25 ns cache. It's just that the technology matured and became cheaper, so the manufacturers have started using 15ns cache more. Also keep in mind that unlike its predecessors, 386DX-40 was a budget-oriented CPU, especially in 1993-1996, so 386DX-40 motherboards generally had cheaper components.

Btw, I also have a board that features original 64K-20 modules and has options for either 64Kb or 256Kb of cache. Generally proving the same point - 386DX-40 came with cheaper components.

- IIRC the tag RAM chip(s) needs a faster access time than the other chips. Of course it doesn't hurt if all chips have the same short(er) access time.
- At least for 486 some people have posted here about setting cache access times (latency cycles or something like that) in the BIOS setup. I suppose using faster chips may enable you to set faster values in the BIOS setup which hopefully will result in faster performance. Don't know if the BIOS setup in 386 boards allows you to do this. If there is no such setting you could try AMISET (in case the setting is just hidden), or use some chipset specific tuning tool.
- In case one of the 386DX-40 boards is the one I sent you, I'm pretty sure I upgraded the cache so the chips may well be faster than the original ones.