Here's a very rough breakdown of the operations it does:
1.33 s 56.7% specialized Collection.split(separator:maxSplits:omittingEmptySubsequences:)
263.00 ms 11.2% Substring.lowercased()
226.00 ms 9.6% specialized Dictionary.subscript.modify
189.00 ms 8.0% readLine(strippingNewline:)
As you can see, calling split on the string is really slow. The reason it is slow is that it's using the generic implementation from Collection, rather than String, which doesn't really know anything about how String works. So to do the split it's doing a linear march down the string calling formIndex(after:), then subscripting to see if there's a space character there using Unicode-aware string comparison on that one Character.
Swift is really nice that it gives you "default" implementations of things for free if you conform to the right things in the protocol hierarchy. But, and this is kind of unfortunately a common bottleneck, if you "know" more you should really specialize the implementation to use a more optimized path that can use all the context that is available. For example, in this case a "split" implementation should really do its own substring matching and splitting rather than having Collection call it through its slow sequential indexing API. Probably something for the stdlib to look at, I guess.
The rest of the things are also not too unfamiliar but I'll go over them one by one; lowercased() is slow because it creates a new Substring and then a new String, plus Unicode stuff. Dictionary accesses are slow because Swift uses a secure but not very performant hash function. readLine is slow because the lines are short and the call to getline is not particularly optimized on macOS, reallocs and locks internally, then the result is used to create a new String with the newline sliced off.