Earlier quoted context omitted.
Xcode with Swift is a bit odd in that its performance varies greatly depending on project structure and as nonintuitive as it may be, the way the developer writes code. For instance a sprawling project with an even split of Objective-C and Swift that constantly call into each other can bring SourceKit to its knees, as can some CocoaPods setups. Going Swift-only or even just reducing the surface between Objective-C an…
To summarize, this is the most Apple of arguments, that you're making: "You're holding it wrong."
I believe the parent was saying that the performance of xcode varies based on the project. This is true - breaking projects up into more modular setups has long optimized project build speeds. The quality you get out of your tools is in many cases based on how you use them.
"You're holding it wrong" would apply if you were deliberately using the tools in a sub-optimal way to try to prove a point.