I tested on both the VM and the bare-metal Win10 system and I can
confirm severe performance issues with the new 'gdi+soft' branch.
Anti-aliased line drawing comes with a price.

However, I believe that there may be many applications that don't suffer
from this performance penalty and thus my vote is

+1 on integration of the new 'gdi+soft' branch

Should 'gdi+' be the default?

I'm not sure, but I suggest to make it the default for 1.4.x development
so it can easily be tested and will be the default in snapshots that
users may test. I agree with Greg (IIRC) that this will be a good way to
get feedback from users.

Depending on the feedback (or no feedback) we may reconsider the default
(GDI or GDI+) before we release FLTK 1.4.

I'd also like to see an option to switch drivers (GDI <-> GDI+)
dynamically so users can maybe switch back to pure old GDI in
time-critical drawing paths (somebody mentioned this, IIRC). This option
might already exist (internally) but I don't know if there's an API to
be easily applied. If so, it should be documented.