size_t for length on x64 will make app slower than on x86?

[FrankLike via Digitalmars-d]

Many old projects need move from x86 to x64,but the 'length' type 
is size_t,it will change on x64,so a lot of work must to do.but I 
find some info which is help for d:
http://www.dotnetperls.com/array-length.
it means:
  test length and longlength, and found 'test longlength' is  
slower than 'test length'.

  0.64 ns   Length
  2.55 ns   LongLength

I love D.So I don't want my app on x64 slower than on x86.

Hope change in 2.067.

Thank you all.