Yes the index was just on the groups field, no it wasn't up to date. The problem went away when I brought the index up to date. What's up with that, do you know?
The default langc settings do not include digits, so a substring search is done searching linearly. You could do the following to bring the linear search behaviour in line with the index behaviour:
set langc='[\alnum\'']';
set wordc='[\alnum\''\-]';