I've modified the warning line a little, and it looks like this:

warn "Scanning and caching *.c files... (for #{meth}, in #{gem_dir})"

The output:

[7] pry(main)> Scanning and caching *.c files... (for #<UnboundMethod: Kernel#is_a?>, in /home/gutov/.rbenv/versions/1.9.3-p429)

Needless to say, it takes a while (>20 seconds on the first invocation), which is particularly annoying in my use case: providing automatic calltips for Ruby code in Emacs.

[12] pry(main)> PryDoc::VERSION
=> "0.4.6"