Hi
First step here is investigate the path this file does and see if there´s any low bandwidth link. Then, if you are sure that from one end to the other you have only high speed links, then you should worry about any device that may filter this, like firewall for example. Some device may inspect this traffic causing low speed data rates. Lastly I´d take a look on the device where the IOS is stored. If this device is too busy, it may reduce transfer speed. And lastly, I´d take a look on the switch which will receive the IOS. You can try transfer to different devices and also try different sources as well.
