The exponential issue was still there in version 4 and I think 5 as well. Didn't say SourceKit crashed, but it was virtually useless because it functioned like a crap version of the language parser.
It helps if you test declare it like:
let json: [String: Any] = [ /** lots of fields here */ ]
I'm not telling you're crazy, I just noticed that things got so much faster in the past two years.