Thank you for the suggestion. However, after inspecting fstab, it appears that the installer had already enabled log on /. (Just for fun, I turned it off, rebooted, and indeed, performance decreased even further when writing small files, by a factor of 5.)
I do not think the problem is related to any filesystem (ffs included), since long, sequential writes (file > 100 MB) to either ffs and ext2 in NetBSD suffer degraded performance (both cases identically give ~10 MB/s, compared to ~25 MB/s when writing to ext2 in Linux. Although, when writing to small files in NetBSD, the performance degradation is indeed amplified even further.
What is puzzling is: if the performance loss is not likely to be related to any filesystem implementation / option, one logical hypothesis is that the partitions are not aligned to 4k blocks, but this has already been falsified by booting Linux and observing better performance for the exact same ext2 partition. Indeed, when issuing a misaligned dd write in Linux, performance then does abruptly decrease.
I can only think that the loss of performance is related to the NetBSD kernel's idea of the entire disk (/dev/wd0). E.g., dma is not being utilized, or maximum sata data rates are not taken advantage of, etc. This is also strange, since the machine is not new (it is from 2009).