For a signal declaration like so:
signal clicked(color cellColor)

and signal handler like so:
onClicked:

{ helloText.color = cellColor; }

QML will generate a function expression

function() { helloText.color = cellColor; }

and the resolution of the arguments (cellColor, in this case) is done dynamically in the QML scope.
Instead, QML could generate the expression

function(cellColor)

{ helloText.color = cellColor; }

and pass the arguments directly to QScriptValue::call().
This should make arguments access a lot faster since it allows JSC to use "static" opcodes (i.e. avoids virtual calls to scope chain).