Some possible causes are network latency, webdirect server configuration, and webdirect file optimization.
Google filemaker web direct optimization and youll see some videos that address how to optimize a file for webdirect.
Compare your FileMaker server deployment with reccomendations from FM. This will reveal whether you need to invest in hardware or change your configuration (move to separate webD server for example).
Fixing your network is not something i can comment on.