In addition to the KeepAlive trick which is a "server-side" modification, on the "client side" default_socket_timeout should be increased from its default value (60) when you deal with ~slow SOAP servers.
As for the KeepAlive, if creating a new separate vhost for the soap api is not possible, you can add this to your existing vhost: BrowserMatch "^PHP-SOAP" nokeepalive
where PHP-SOAP is the agent name of your soap client, if you dont know what agent name your client use, just checkout the access.log of your apache.
When you get errors like:
"Fatal error: Uncaught SoapFault exception: [HTTP] Error Fetching http headers in"
after a few (time intensive) SOAP-Calls, check your webserver-config.
Sometimes the webservers "KeepAlive"-Setting tends to result in this error. For SOAP-Environments I recommend you to disable KeepAlive.
Hint: It might be tricky to create a dedicated vhost for your SOAP-Gateways and disable keepalive just for this vhost because for normal webpages Keepalive is a nice speed-boost.