You could a reviver function of JSON.parse and convert numerical values to numbers.

Using the reviver parameter

If a reviver is specified, the value computed by parsing is transformed before being returned. Specifically, the computed value and all its properties (beginning with the most nested properties and proceeding to the original value itself) are individually run through the reviver. Then it is called, with the object containing the property being processed as this, and with the property name as a string, and the property value as arguments. If the reviver function returns undefined (or returns no value, for example, if execution falls off the end of the function), the property is deleted from the object. Otherwise, the property is redefined to be the return value.

If the reviver only transforms some values and not others, be certain to return all untransformed values as-is, otherwise they will be deleted from the resulting object.

var json = '[{"key":"Idle Time","values":[{"x":"SADSA","y":"4.0"},{"x":"FDDG","y":"6.0"},{"x":"FF","y":"4.15"}]},{"key":"Operational Time","values":[{"x":"SADSA","y":"20.0"},{"x":"FDDG","y":"18.0"},{"x":"FF","y":"19.45"}]}]',
    data = JSON.parse(json, (key, value) => !isNaN(value) ? +value : value);
    
console.log(data);

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