If you're writing an XML-RPC client which makes lots of small function calls, you may discover that your round-trip time is fairly high, thanks to Internet backbone latency. Some servers allow you to batch up multiple requests using the following function:
array system.multicall (array calls)
You can find more information in the system.multicall RFC.
This method is currently supported by servers written in C and UserLand Frontier. Servers written in Python and Perl can use the code at the XML-RPC Hacks page.