> Right. One of several bits of architectural friction that didn’t matter while your large DBMS was I/O constrained.

Yep. Lots of architectural design out there based on IO latency being higher by an order of magnitude or two than now, while memory latency only shrank modestly.

To be clear, using one process with loads of threads has its own set of issues. Much easier to hit contention in the kernel, e.g. the infamous mmap_sem in linux.