Project loading is slow for large project

QmlJS ModelManager::defaultProjectInfoForProject() resolves mimetype for each file. When I open a qbs file with ~20K files (in which only the qbs files are qml-type), it is pretty slow. Callgrind shows that it takes ~28% of the CPU time for this thread.

C++ resolving performance was improved in 8c90998fff6ec3cf85ad87b56175e86b6f0a93d0 but QmlJS remained the same.