Some slowness can be expected as Xcode attaches extra processes to your app/device for debugging, though here's a couple things to try.
Try setting your build config from the Product menu > Option+click Run and set the build configuration to Release and see if that helps.
Instruments is a developer tool a part of Xcode that lets you test your app for memory leaks etc. You access it from the Xcode menu > Open Developer Tools > Instruments. It's probably beyond the scope of this answer but check out this Hacking With Swift article https://www.hackingwithswift.com/read/30/3/what-can-instruments-tell-us