Script Timeout and Dialup connection

[SPD]

I've been researching this issue with little luck. Here's the situation:

We have a few users on slow dialup connections who are posting data from
forms. Frequently, they get script timeout errors. We've gotten timeouts
from users with faster connections as well, so I suspected that the overall
load is overtaxing the server. But it does seem to be more frequent with
dialup users posting relatively large (>10K) worth of data from forms. The
Script Timeout value for the site is 20 seconds. IMO, it's seems quite odd
that connection speed could influence a server-side script timeout, but...

Question: When form data is posted, does the server begin processing upon
receipt of the first packet with the post request, or does it wait until all
packets have arrived?

Question 2: Is 20 seconds too short, or are there potential problems with
increasing it to, say, 30, 45, or even 60 seconds?

Any help would be appreciated. Thanks.

Scott