Repeat for each line of a variable or field?

[David Epstein]

I've done a little bit of speed testing showing extremely slight differences, but wonder if anyone who understands the "engine" can advise on this:

Is there a difference between these two approaches?

(1) put fld 1 into txt; repeat for each line k in txt; etc.

and
(2) repeat for each line k in fld 1; etc.

For no very good reason, I tend to do (1), but have begun to wonder:  for a very lengthy field 1, does my writing to a variable use up more RAM, and so perhaps injure performance?  Or is the engine in effect writing to a variable either way?  Is a variable read more quickly than a field?  

Many thanks.
David Epstein