Is there any difference in download time of a compressed file and an uncompressed file, both of same size? It may sound silly, but I haven't yet tried it and don't know if there is any difference.
If there is no additional encoding or compression of the data stream as part of the network then no.
Dial up modems typically enable compression (seriously speeding up downloads of uncompressed data), and the extra complexity was worth the large data rate improvements. These days, if not on dial up. such DCE-DCE compression is unlikely so no is the likely answer.
Also note that such in-network compression is likely to give poorer results that the kind of compressions available that are format away as it has to be general purpose and lossless.