I generally found StringStream to be kind of slow (that is why I made the flatty/binny library).
But really what you want to do is profile, profile, profile. If this is not on a hot path it does not matter how slow it is. I recommend vTune. I have had a ton of success with vTune. Just compile your nim file with --debugger:native and run your exe with vTune. After a bit of getting used to it tells you where the hot paths are and how long stuff takes.
