Efficiency question for list modification

[FlexibleLearning]

Problem:
I have a long list of several thousand lines.
Each line contains two comma-separated numbers.
I want to divide the first item of each line by one divisor, and divide the
second item of each line by a different divisor.
The list order must stay the same.

Example:
Using 2 and 5 as divisors...
  10,10
  12,15
  8,12
would become
  5,2
  6,3
  4,2.4

Options:
Using "repeat with n=1 to num of lines" takes far too long.
Using "repeat for each line L" either attempts to modify read-only data, or
is only 25% faster using a dumping variable.
Using split/combine will mess up the ordering (numeric array keys are not
sorted numerically with combine).

Any other ideas?

Hugh Senior
FLCo