Will 16GB microSD card slows down LG p500 Optimus one ?

It won't slow your phone down. Putting 16 GB in is a good thing. My brother has this phone but only has a 2 GB MicroSD card and he has less than 30 MB left on the card (too much stuff). It's a pretty good phone. Acutally, it's the best phone you could find in this price range!