Check to see what speed the serial port is running at. It should be 115K. If it is set higher than that it slows down the connection, a lot. No idea why, I've ran into that severeal times with Kppp and wvdial.
On the difference in 21.6 and 9, keep in mind some use bytes and some use bits. It depends on how it is measured. When I was on dial-up I connected at 26.4K but only got about 4 to 5KBs through put, on a good day. That may be normal, may be a setting to adjust.
Hope that helps.