if you are using real-time translation the performance is impacted. You can assume that a paragraph translation takes anywhere between 500 milliseconds and a few seconds.
Placing the Microsoft Translator widget on your site does not impact rendering of the original language, but the rendering time when user chooses to perform the actual translation will be longer, compared to no translation happening.
You can avoid a rendering delay altogether if you translate and publish the translation on your site.
If you need more detail, please describe your situation.