Looks like you have a strong machine, so you can increase some of the defaults for tagging performance. Here are a few suggestions:
1) First, if you have had to debug background tagging before,
you might have turned on the macro variable (def_tagging_logging).
Macro > Set Macro Variable... > def_tagging_logging (Set it to back to 0).
Leaving this on does a lot of logging which can drag down performance.
2) Tools > Options > Editing > Background Tagging...
Use background tagging threads: Make sure this is ON
Number of tagging threads to start: Increase to 8
Max number of active tagging jobs: Increase to 5000
Max amount of background tagging memory usage (MB): Increase to 128
3) Tools > Options > Editing > Context Tagging(R)
Use Memory Mapped Files: Make sure this is ON
Tag File Cache Size: Increase to 64M (you will have to turn Memory Map off temporarily)
Use independent database file caches: ON
4) While rebuilding the tag file in the background, your CPU usage should stay high.
SlickEdit does throttle back the background tagging a bit, to be nice to other processes
and to make it possible for SlickEdit to keep rocking in the foreground.
If it is dragging down into the 10% range, then you are probably at the point where the
database writer thread can not keep up with the parsing threads.
5) Make sure you are getting the performance you think you are from your SSD.
If it in an external USB drive, make sure it is on a USB 3.0 port.
Run the BlackMagic speed test to verify the throughput.
If your SSD is software encrypted, you might also be losing some
throughput on account of that.
6) I experimented at one point with using a RAM drive and directing my tag files there.
It was very fast, except for the fact that it used a lot of memory and cleared itself
every time I logged off. That was actually not a bad thing, but with a large enough
project it could be kind of a setback. That combined with the fact that the RAM drive
was redundant with the internal caching using memory mapped I/O.
A couple questions about your project:
1) How large is the tag file after the build is finished? Tools > Tag Files...
2) Does it make heavy use of the Boost preprocessing macros?