There is not a place on the Mac, and I suspect other machine too, that will tell you how fast your DSL modem is pumping out.
You could try a Google search for Broadband speed tests, any of which will show your speed from end to end. Bear in mind that your ISP will never guarantee your upload/download speed and many factors come into play when you start trying to figure out your speed.
Your DSL modem is talking to your Mac at 10MB, regardless of what your NIC is pumping out.
For instance, you have a 5MB up and down speed. (far fetched to bear with me) That speed is probably only reached when you are talking directly to the other modem, the one at the ISP.
If you decide to download a megga file from somewhere else, you have to bear in mind all the connections necessary to reach that place. Any one of those links could be the cause of what appears to be a slow down. The speed of your download is totally dependent on the speed of the SLOWEST link.
You could try downloading a program (utility) called iStat Menu's which will show you the data flow to and from your Mac.
P
where on the mac can you determine how fast your dsl modem is pumping out? I suspect mine has slowed down but I don't know where to find the speed.

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