How about selling the value of what clicking +1 will do for the user? +1 is a new service so the features are low but it’s certainly going to be a way to push updates out, interact, even poll users on features. Give me a reason to engage & value from doing so.
It will slow down any page so this logic would indicate you wouldn’t want it anywhere… right?
Of course for sites that do find a significant drop in profit per tenth of second slow down, there are services that handle tag loading as it’s not just social sharing: analytics, ad, retargetting all pose the same issue.
Basically, there are various lines but the most often used line is “rate us on Google”. This line is best for you if you are adding it with +1 button.
Just be sure you’re not misleading people into thinking they’re actually providing a review when they’re actually sharing [which is an implicit endorsement but not necessarily what they may think of as reviewing].
Webcosmo already helped in this matter, Thanks to webcosmo. It is true that +1 button makes your website a bit slow, however, as per a statement by Google, +1 button is working 3-times faster, i.e you may not get the rank you want. To cope with slow page download, you could use gzip compression technique. It won’t make your website slow.