I find that Xcode is contantly indexing - runs through indexing the code, then does it again.... and again.... and again...
Surely all this read/writing will kill people's SSD's, not to mention batteries.
Constantly indexing
Seen this SO thread? Might be worth a try if it fits your example...
http://stackoverflow.com/questions/39547197/xcode-8-0-swift-3-0-slow-indexing-and-building