do you have anything running in background on the tablet? Maybe something you are not aware of? After the bandwidth thing the next think I would think to check is CPU load when Skype is running then when Skype is running.

There was something I read about here that can run w/o a person knowing but for the life of me I can't recall (these tablets are all new to me...and I am lucky I can find the on button.)

Wish I have more of a suggestion because like you I use Skype exclusively as my out going line and as a cellular replacement when out and about (I use EVDO RevA and a bunch of hardware that fits in a small pouch but that is another story...)

Also, your ISP might be throttling anything P2P and their system could think Skype is a P2P app thus throttling it's use and speed on purpose.

For me Skype has been fine but that means nothing for your situation.