Use the least amount of I/O possible—read the entire file into memory, process it in a single pass, and write back once. Replace any explicit loops with vectorized operations or built‑in map/reduce functions; they’re usually 5‑10× faster. If the data is large, consider chunking and a thread pool or asyncio to overlap I/O with CPU work. Profile first—measure the hot spots with a simple stopwatch or cProfile—then target those. And remember, a cleaner algorithm often beats a fancy micro‑optimization.