Mr. Happy,

What I said about songs in my post also applies to purely instrumental tunes. Musically a song is simply music with words. Your voice is an instrument and if you are singing a song, you are also singing a tune

I mentioned song as an example where I had noticed the issue most, but what I said applies just as much to tunes. If you are nervous it is not uncommon you will lead off too fast - even for yourself.

As someone who both sings and plays I do not see any fundamental difference between songs and tunes.