man im sure, and i cleaned the cache and what im saying in true u can try it ur self.
download a file from IE without any download manager.
delete your browser cache, redownload the same file again in the same way(no download manager) , fast download.
delete again ur IE cache and download the same file with any download manager .the download will be normal speed because it has been removed from cache(the cache will become for example CACHE-10MB if the file is 10MB) and i am really sure what im saying.
THIS thing was not true in the old mikrotik(I MEAN IN SQUID). download from download managers used to get files fast from cache if it was cached(OF course files downloaded from the first time from a download manager are not cached, it connects to many servers so no cache)
.Thx and sorry if someone did not understand on my sentenses