Using CFLAGS=-O2 seems to have cleared up the clang segfault problems for buildworld,
so the next test is to try rebuilding chromium. A make clean was run in www/chromium
but the same sources used previously were retained.
Swap consists of
/dev/mmcsd0s2b    4404252    45764  4358488     1%
/dev/da0p5        2097152    46204  2050948     2%
/dev/da1p2        3145728    46500  3099228     1%
at the moment. da0 is a 64GB Sandisk SDCZ80 usb flash drive,
da1 is an old 32 GB hard disk in a USB case. Both are connected via
an Amazon Essentials USB hub with separate power supply. There is no
keyboard/mouse, but there is an HDMI monitor connected. 

CFLAGS=-O2 is in /etc/make.conf, but nothing else. 
/boot/loader.conf contains 
vm.pageout_oom_seq="2048" to prevent OOMA from getting impatient

After four days the system reached a little past the halfway point in the job count.
Swap usage is running 2-3 GB, with idle fraction in the 80-90% range for the most
part. It looks as if the mechanical hard drive is the main bottleneck, showing
over 90% busy while the USB flash and microSD swap partitions are below .1% even 
though swap usage is spread evenly across all three.

There was one segfault, somewhat past the halfway point, but make
restarted without issue and ran to completion in about eleven days.
Swap use peaked at 3337744 MB, about 35% of that configured.

The resulting binary installed but didn't run, failing on a library error:
ld-elf.so.1: /usr/local/lib/libglib-2.0.so.0: Undefined symbol "environ"

This has apparently been fixed in www/chromium, so ports are updated and
make is running again. It looks like a complete start-over, so it'll be
another week and a half.