Why does setting input value take 600ms?

Question

I an setting value for an input tag in firefox.

If I set it to a literal string, there is no delay. The delay only happens when I try to set the value to that of an object property (contains a string), then it takes 600ms.

a)

document.getElementById('inputtag').value='test' // No delay

b)

object.testvalue='test'
document.getElementById('inputtag').value=object.testvalue // 600ms delay