From Tonec’s official changelog for 6.25:
Observation: On a fast, uncongested connection, IDM’s segmentation did improve single-file speed due to TCP fairness algorithms and router buffer bloat. The main benefit was parallel downloads and resuming broken transfers.
Unlike basic downloaders that divide a file into 2-3 parts, IDM 6.25 Build 20 can split downloads into up to . As the download progresses, the engine tests the server’s response and adjusts segment sizes to optimize throughput. This results in maximum bandwidth utilization without overwhelming the network.