c:\coding>ghc test.hs
[1 of 1] Compiling Main             ( test.hs, test.o )
Linking test.exe ...

c:\coding>test 10000000
"Prelude version"
90.98579564908658
CPU time:   9.81s
"Vector version"
90.98579564908658
CPU time:  10.75s

c:\coding>ghc test.hs -O2   -- nothing actually happens

c:\coding>test 10000000 -O2
"Prelude version"
90.98579564908658
CPU time:   9.80s
"Vector version"
90.98579564908658
CPU time:  10.92s