Comment 2 for bug 1675357

This is a `perf report` from just 3-4s around reactivating a unity8 vt:

  18,33% Mir/Input Reade libc-2.24.so [.] malloc
   5,42% unity8 libQt5Core.so.5.7.1 [.] _ZL28qMetaTypeCustomType_unlockedPKciPi.constprop.817
   4,55% MirServerThread libglib-2.0.so.0.5105.0 [.] g_main_context_dispatch
   3,44% unity8 libQt5Qml.so.5.7.1 [.] _ZN3QV413InternalClass4findEPKNS_6StringE
   2,34% unity8 libQt5Qml.so.5.7.1 [.] _ZNK3QV414QObjectWrapper12findPropertyEPNS_15ExecutionEngineEP15QQmlContextDataPNS_6StringENS0_12RevisionModeEP16QQmlPropertyData
   2,31% unity8 libQt5Qml.so.5.7.1 [.] _ZN3QV413MemoryManager9allocDataEmm
   1,83% unity8 libc-2.24.so [.] _int_free
   1,56% unity8 libQt5Qml.so.5.7.1 [.] _ZN11QQmlBinding6updateE6QFlagsIN19QQmlPropertyPrivate9WriteFlagEE