Why do downloads (storedownloadd) from the Mac App Store consume so much CPU?
Ran into the same issue today when updating XCode: storedownloadd pegs a CPU core for 20 minutes. 

I tried breaking into  storedownloadd with a debugger and saw some really long stack traces and a lot of time spent inside Security::CodeSigning.

I suspect that it is recursively verifying the digital signatures of a lot of tiny files one-by-one. I have no idea why it would do that instead of verifying the whole package as it is being downloaded. I also see a lot of string manipulation inside Security_CodeSigning::RequirementLexer::RequirementLexer. 

Whatever storedownloadd does, it is too damn slow!