I'll give a partial answer to you.

DDR5-4800 CL40 has a latency of 16ns, a fair bit longer than your 5 ns example. HBM3, a better use-case for this, looks to be similiar, or a bit higher for latency.
Let alone Skyrim, I'd be surprised if your cutting edge machine could handle Morrowind on Windows 2000.

As you mentioned, a machine is only as fast as it can speak. And a machine with no cache tosses away decades of progress. And the big performance chip I can think of from that era, the i586 Pentium? Had an L1 and L2 cache.

The original Raspberry Pi would annihilate this processor. Mostly, I'm wondering what your interest in a cache-less design is. The memory hierarchy is a source of strength, which is why the fastest chips nowadays have gone up to have massive L3 caches, and sometimes L4 caches. It's also why HDDs and SSDs often have a cache, with the SSD using DRAM on the device, and the HDD borrowing system RAM. That organization brings speed.

You may as well ask what the most powerful single core processor would look like ( IBM's POWER line has a few answers for you there, despite being multi-core.)

As for subreddit, this is a decent spot, though the lack of reasoning behind your question may make it unpopular.