Unless you are buying some external DSP hardware like a Pro Tools rig which does all the heavy lifting for the Mac (which is how big studios get away with using under powered Macs for years) adding an audio interface is not likely to improve the situation with the possible exception of using any DSP effects instead of plugins that require native processing (that's where the computer's CPU actually has to do the work) in fact any additional inputs and outputs will require additional resources which is why when using native processing it's a good idea to disable any unused inputs and outputs, oh yeah and if you use USB that's going to tax the CPU a bit as well, now having said that it might still be beneficial to attach a second hard drive to your awful inside-out craptop computer so that you can stream audio, ideally a music computer should have a separate hard drive for streaming audio and one of the benefits is it allows you to make more efficient use of the freeze function which frees up both memory and CPU usage, basically hitting freeze renders a hidden audio track from a plugin so that it can be streamed from a hard drive instead, the plugin being temporarily switched off to conserve memory and CPU usage.