Txs, mudmanc4! I appreciate yout attempt to help me out!
No, i did not connect them at the same time. My DSL link is still available, so in order to compare speeds, i just unplug the cable modem wire (RJ45 in the network adapter) and plug in the DSL modem cable in the same adapter. I did that just to check why almost everything is so slow in cable, then i figured out that it only gets to download fast when there are plenty of sessions passing data (like when using a download manager software). Browsing, watching videos or speed testing is quite sluggish all the time!
As an example, if i watch a YouTube HD video, it tops out at something like 120KB/s (which is not enough to buffer pretty much any HD video in there). However, if i pop up like 5 different HD videos, it totally use all the power this link offers (~480KB/s). I know it's not some YouTube limitation because i can get 260KB/s from a single video when using DSL.
Another example: Right now i'm downloading a random linux distro (mandriva) from a server with FDM at 480KB/s. But if close download manager and start downloading from a browser like firefox of chrome, it will not download faster than 70KB/s unless i unplug the cable link and plug in the DSL link. In this case, it doesn't matter which application i use to download the file, it's always gonna use all that this 2.5mbps DSL is able to deliver (something like 240KB/s).
Weird, huh?