I recently upgraded from Tiger 10.4.11 to Leopard 10.5.6. Everything seemed fine, except for a drastic slowdown in my dial-up internet access via the internal modem. Oddly Safari and Firefox ran fine, but my Mail and FTP/Telnet were crawling under the new OS.

The solution that worked for me was to uncheck the "Use TCP header compression" setting, which can be found here: System Preferences » Network » Internal Modem » Advanced button (lower right) » PPP tab. On that tab, set the Settings pop-up menu to Configuration, and uncheck the "Use TCP header compression" box. Then click the OK button in the lower right, followed by the Apply button (in the lower right), and you're done.

If your modem is connected, disconnect and reconnect to implement the change.