Why the given expression takes so long in IRB

Any idea why the following simple expression lasts so long in IRB:

irb(main):003:0> res='a'*(10**6)

It runs several minutes before finishes.

Whereas suppling the same expression to puts method f.E. is instanteous:

irb(main):003:0> p开发者_如何学编程uts('a'*(10**6))

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I just figured out it's caused by Wirb module loaded from .irbrc startup file. By commenting it out both examples now run comparable fast.