i think you nailed the centrail point.

rotational disks usually top up at ~85 MB/sec, with seek time being up to 12 msec for consumer drives and ~6 msec for enterprise drives (15k rpm).

ssd could saturate the sata bus and would top at 500-550 MB/sec, with essentially no seek time. latency wold be anything between 50 and 250 microseconds (depending on the operation).

nvme disks instead can sometimes full utilise a pci-e lane and reach multiple gigabytes/second in sequential read (ie: pci-e gen5 nvme disks can peak at 7 GB/sec) with latencies as low as 10–30 microseconds for reads and 20–100 microseconds for writes.

As compiling the kernel meant/means doing a lot of i/o on small files, you can see why disk access is a huge factor.

A friend of mine did work on LLVM for his PhD... The first thing he did when he got funding for his phd was getting a laptop with as much memory as possible (i think it was 64gb on a dell workstation, at the time) and mount his work directory in tmpfs.