I use fs.readFile and fs.writeFile a lot, often repeatedly reading/writing the same file paths.
Would it be better for performance to open each file with fs.open once, and then reuse the same fd repeatedly (with fs.read or fs.write)?
Opening a file is a very, very expensive process. You are best served by keeping the file descriptor active and then just read/write at will. The only time you need depart from this is when you run short of file descriptors.