This has already discussed on the "interest" mailing list ("font smoothing gamma changes in Qt 5.9?"), I'm mainly logging a ticket here to have an easier reference anchor and to attach a comparative screenshot.

Changes in font stem darkening introduced in Qt 5.9 make (certain) fonts look severely watered down under X11, at least when using Freetype 2.6.5 or 2.7.1 . I first thought this was some kind of interaction with the Infinality patches I apply to Freetype but this does not appear to be the case.

Screenshot showing in the left column stock Qt font smoothing gamma (1.7) vs. tweaked gamma (0.95) in the right column; top row is Freetype 2.71 stock, bottom is Freetype 2.7.1 + Infinality. 

The Infinality bundle does come with a set of hand-tuned config files for FontConfig, and these do appear to explain the difference I see for fonts I've tested. The screenshot above is made with FontConfig+Infinality, below is a comparison without that part of the Infinality Bundle applied (Freetype does have the patches applied here):

Left: FontConfig +Infinality, fsmgamma=0.95; right Freetype +Infinality, FontConfig stock, fsmgamma=0.95 .

I hesitate to call this a bug but it should be noted that Qt apps are now the odd duck in the font pond: I'm not aware that the hand-tuned fontconfig files cause issues with any other rendering engine instead of providing an optimal rendering quality. Would it be acceptable to make the gamma tuneable via a QPA argument and/or env. variable?

(the other  tracker categories don't seem to fit any better though so I stuck with "bug" and will let you determine the priority).