REG: performance regression in qml delegate "label"

A 24% performance regression has been seen in qmlbench between 5.12 and 5.15.

It appears this entered the codebase in 5.14 sometime around 02-Dec-2019 and was subsequently merged up to dev/5.15.

While there is a single large drop, 5.14 was getting slower over time prior to the change, which should also be investigated.

See https://testresults.qt.io/grafana/d/5ei7yHViz/qmlbench-unlocked-dashboard?viewPanel=200&orgId=1&var-suite=All&var-benchmark=All&var-branch=5.15&var-branch=5.14&var-branch=5.13&var-branch=5.12&var-branch=dev&var-hardwareId=Ubuntu_16.04_Foucault_x64&from=1567743028665&to=1582095773442