Reading a big file into a list can kill your program. A generator handles one line at a time. See the memory difference measured on a real file, learn what yield does, and chain small generators into a pipeline.
asyncio.gather runs many waiting tasks at once in one thread and returns results in order. Avoid await in a loop and blocking calls, keep failures with return_exceptions=True, and limit concurrency with a Semaphore.
Don't guess which part is slow. Run cProfile, read cumtime and tottime the right way, fix the one line that matters, and measure again. In the example, the obvious suspect isn't the problem.
for line in file reads one line at a time, so a 10 GB file uses almost no memory. read() and readlines() load everything. Count, filter and stop early without building a list, and read in blocks when there are no lines.
Downloads spend most of their time waiting. ThreadPoolExecutor lets many waits overlap: 20 half-second downloads in 2 seconds instead of 10. Handle failures with as_completed, pick a sensible worker count and respect the server.