The "Tower PC" was using its main SSD for the Bitcoin database but initially, the laptop PC was also using its SSD (and its faster than the one in the Tower PC) and it was just as slow on the laptop as when using the NAS.

The NAS has write speeds similar to what many people have in their PCs unless they have SSDs, so that's probably not the issue.

One thing I noticed - the Tower PC CPU usage was around 30% and it did not feel overloaded at all. I could browse the net, for example, without noticing any performance degradation. On the less powerful laptop PC, the CPU usage was only around 5-7% according to task manager, but it felt much slower than that. Badly managed hyperthreading maybe?

Really odd.