When QFontMetrics class methods (and maybe a lot of more stuff as the problem goes down to QFontPrivate) are used in an multithreaded environment (for example using QtConcurrent), there is a font database (re)load on every thread context switch! At least on Linux/X11 this is extremely slow and makes the application perform by orders of magnitude slower than without multithreading.

The root of the problem is QFontPrivate::engineForScript(int script) const, respectively the global QFontCache it uses and drops on every thread context switch.