The time to open and close the files is completely insignificant compared to the time to read them, especially if the files are large. You really don't want to do it that way!

Tell us about the bigger picture. It is almost certain that there is a better way of achieving what you want to do than reparsing one of the files once for each line of another file. It may be that you need to sort the files first, or use a database, or extract key information, but whatever the technique, it will be much faster than the scheme you are currently considering.