I think if we can match swf size with original text width's then rendering will be fast.
The font-replacement flash generated by sIFR takes the same space as the replaced HTML element. So if for example the text you are replacing is inside of an h1 tag, sIFR will take up the whole space of that h1 element and not the space of the text inside of it.
I guess the element that you are replacing has a width of 100%. You can set a border on it (style="border: solid 1px black;") so you can see the bounds of the element.