Data::Dumper does by default replace all callbacks with sub { "DUMMY" }. Most likely, these callbacks wait for some results from Firefox.

If you have a sample script and the website is somewhat public, I can try to reproduce the slowness.

In reply to Re^5: WWW::Mechanize::Firefox delayed returns / slow by Corion
in thread WWW::Mechanize::Firefox delayed returns / slow by tcordes

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