I'm about to buy a external monitor to Macbook pro 13 mid 2010, I really like Dell U2515H, can my mac support it without being slow ? I will use it for developing (sublime text) and Videos/Movies.
Your Macbook supports an external monitor with a max resolution of 2560x1600. It will be fine, for better performance, close the lid so that there is only 1 display open.
I had 2x Thunderbolt displays at 2560x14400 (13" Late 2011) and was only experiencing slow movements after I plugged in the 2nd display. 1 will run just fine. :)
Check the inputs. If the monitor supports video in through HDMI, it'll work great with an MBP (note that you'll need an adapter for MBA).