Sometimes traditional Common Lisp implementation techniques compare so poorly to the techniques used in other languages that Common Lisp can become an impractical language choice. Terrible inefficiencies appear in number-crunching programs, since Common Lisp numeric operations often involve number-consing and generic arithmetic. Python supports efficient natural representations for numbers (and some other types), and allows these efficient representations to be used in more contexts. Python also provides good efficiency notes that warn when a crucial declaration is missing.
See section non-descriptor for more about object representations and numeric types. Also see efficiency-notes about efficiency notes.