I agree with ikalo, it is just the buffering thing that make it as if it was faster at start. It is base on the amount of data written to the hardisk/ amount of time taken. First few block usually big and within short period of time.

I don't really believe in download speedup program. I believe for these program to really works, some kind of compression have to take place (or proxy). The tranmitting end must compress the data before sending while your speedup program understand how to decompress it and save it to a file. Unless it is using bit-torrent type of proxy.