Mixing network speeds like this means MacHTTP needs to be tuned to accomodate the variety of networks it will be transmitting to. If MacHTTP is sending data to a client on a PPP connection, connected at 14.4 kbps, this means that it's going to take about 1 second to send 1k of data. DUMP_BUF_SIZE is the configuration parameter that you can adjust to accomodate slower connections. The smaller the DUMP_BUF_SIZE, the faster MacHTTP can shove a buffer of data out to slow client without slowing down all the other connections.
Unfortunately, there is a point of diminishing return, where the smaller DUMP_BUF_SIZE begins to cause MacHTTP to spend more time thrashing through servicing connections than sending data. Usually a value between 512 and 2048 works well for slow clients. If you have a lot of high speed clients, MacHTTP performs better with larger DUMP_BUF_SIZE settings like 4096 or 8192. Experimentation is the order of the day to determine the optimum size.
PIG_DELAY is measured in 60ths of a second. An average setting is 30, or 1/2 of a second. When the server is really busy, MacHTTP runs non-stop for this long before allowing other Mac applications to run. This can cause your applications to appear jerky. Decreasing this number to something like 15 or 20 allows other applications more time to run. Decreasing PIG_DELAY too much will cause MacHTTP to be unable to service its connections at a reasonable level of performance. Experiment!
A more complicated technique is to set up duplicate document trees on multiple Macs, and rely on the domain name service to randomly connect clients to one of these identical servers. You must configure the same domain name for each of these Macs, which have different IP addresses. Newer versions of DNS servers will be able to randomly return different IP addresses for each client requesting the same name.
The basic rule of thumb is to watch the MacHTTP status window (the bar graphs) and adjust the MAX_USERS variable up or down, based on your server's load and performance under that load. Don't be afraid to restrict the number of connections if your server cannot handle the load.