For an adblocker to work properly, it needs to inspect every item on the page you're loading and compare the contents to its own, internal, list of known ad content. This will slow-down your page loading time. By how much it's slowed all depends on the device, how much content (text, images, and ads) are on the page, and the network/internet connection.
You can help the adblocker by white-listing certain sites where you don't mind seeing ads (for instance, to support a site by allowing ads). Each adblocker handles this slightly different so you'll need to refer to the instructions.
AdblockPlus has a blog posting regarding how using their own browser, instead of Safari, allows pages to load a little-bit faster.
You're going to have the same problem regardless of which adblock app you use. I recommend trying a few of them and tweak the settings to see which one works best for you and your device.