[Bug c++/69635] 4x increase of build time [4.9 -> 6.0]

[h2+bugs at fsfe dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69635

--- Comment #3 from h2+bugs at fsfe dot org ---
Thank you for the quick replies!

> Did you build gcc6 with --enable-checking=release ?

I am using the pre-built FreeBSD packages, I have checked, and it seems it is
not the case. That likely explains it... I will double-check with the package
maintainer.

> You can speed up the build time by using -flto=<number of CPU cores>.

Ah, that's very interesting! I hadn't seen -flto with a number yet. Is there
more documentation on it? 
Is this the "WHOPR" mode described here:
https://gcc.gnu.org/onlinedocs/gccint/LTO-Overview.html ?
I had previously thought flto was only useful when linking multiple translation
units... Does it have an effect on the produced binary?