[Interest] slower compilation with Qt6

[Hamish Moffatt]

I've just finished porting my application from Qt 5.15 to 6.5, and I'm 
finding that compilation is significantly slower now - builds take twice 
as long or more. For example a linux-g++ build of a project with ~650 .o 
files has gone from 45 seconds to 100. I see similar increases on 
Windows with MSVC2022, and in other projects with more compilation units.

I'm already using precompiled headers, including a bunch of Qt headers 
and some STL.

The next MSVC2022 preview release has a "Build Insights" feature which 
shows time spent processing header files, and my biggest culprits are 
some headers of mine that use a lot of Q_GADGET and Q_PROPERTY macros.


Does anyone have any suggestions on how to speed things up? I see some 
other comments about this online for example 
https://forum.qt.io/topic/142309/compilation-times-with-qt6/12



Thanks

Hamish