Euler Project 14 (Longest Collatz Sequence)
Using a 3.5 interpreter, things yielding "negative speed-up" included:


@functools.lru_cache(None) (mainly due to explicit base-case handling?)
substituting cache.get() and is not None for in and []
divmod(n, 2) instead of %2 and //2
"inlining the even case"


Negligible speed-up:


Binary operators instead of %2 and //2
Global cache instead of an explicit parameter


Slight speed-up:


rec_col((3*n+1) &gt;&gt; 1)+2


Of 1588206 cache entries, almost half are not used at all, not even one eighth twice, none more often.