The simulation can sometimes give weird results, especially if you multitask while it's testing. Using an image file eliminates having the HD seeking data for the burn, so that's the fastest. Burning lots of small files is the slowest, and this is why there's a setting for "cache files smaller than ..." in Nero. A HD can slow to a crawl when reading lots of small files, like a database. If your drive light stays red clear through the burn, and the buffers indicate full, then you're doing fine.