Tom> To figure this out, we'd probably need a test case.  Maybe comparing
the
Tom> results between the two scenarios with "set debug symtab-create on"
Tom> would indicate something.

I have a reproducer, but it is somewhat tricky to build. It is
also not my program, so I'm not sure if I can simplify it.

The program is KiCad (https://gitlab.com/kicad/code/kicad.git).
Build dependencies for Ubuntu can be obtained from the packaging
files: https://dev-docs.kicad.org/en/build/linux/#_dependencies.

Once KiCad is built I run the following commands in gdb:

(gdb) file /home/nd/w/CLionProjects/kicad/build/debug/pcbnew/pcbnew
(gdb) b /home/nd/w/CLionProjects/kicad/pcbnew/footprint_edit_frame.cpp:114
(gdb) run

When KiCad's window is opened I invoke the menu item "Tools >
Footprint Editor" to hit the created breakpoint, and then run:

(gdb) interpreter-exec mi2 "-var-create var1_this * \"this\""
(gdb) interpreter-exec mi2 "-var-list-children --all-values \"var1_this\"
-1 -1"
(gdb) interpreter-exec mi2 "-var-list-children --all-values
\"var1_this.private\" 0 8"

The last command takes most of the time. The .gdbinit only sets up logging:

set trace-commands on
set logging overwrite on
set logging debugredirect on
set logging enabled on
set debug symtab-create 1

I've captured the logs, they are quite big, the zip (15Mb) is here:
https://drive.google.com/file/d/1Cz-HOOqus6xwf_eIjBqcs727w8aOqco4/view?usp=sharing
.

There are 3 files in archive:

- gdb.txt.index-cache-disabled - the slow run with index-cache
  disabled

- gdb.txt.index-cache-enabled-run1 - the first run with
  index-cache enabled - it is still slow

- gdb.txt.index-cache-enabled-run2 - the second run with
  index-cache enabled - fast