Your mileage will vary of course, but here's some anecdotal numbers...
I'd already optimised the site (W3 Total Cache), all images/resources local and sized/optimised etc.
I then added Social Media buttons (which tacked them on as individual buttons to the bottom of each post) and the page load time DOUBLED. Looking at the waterfall timings every single button was doing remote calls "back home". It was awful. As the number of posts on the page would have increased the strain would have got worse.
This plugin added approx 5% to base page load times which is low enough to actually not be statistically significant due to the testing methodology to be honest (IE the 5% may not be solely due to the plugin).