Between 4.6.0 & 4.6.1, a change was made to qfsfileengine.cpp to "Improve performance of getting the canonical filename on Linux and Symbian by using realpath() system call." (from changes-4.6.1). However, the if defined() check is wrong. On line 147, it's attempting to detect Linux & Symbian. However it checks against Q_OS_LINIX instead of Q_OS_LINUX, thus nullifying the performance improvement.
To fix, change
#if defined(Q_OS_LINIX) || defined(Q_OS_SYMBIAN)
#if defined (Q_OS_LINUX) || defined(Q_OS_SYMBIAN)