The database - imported or pasted graphics, rather than FMP buttons or boxes for interface.
The hosting computer - dedicated FMP hosting or a user machine that is running other apps.
The hosting computer - slow hard drive or badly fragmented hard drive.
Local Network - routers that are 100 mbs (or 1000?)
Local Network - high levels of network activity.
Internet connection - upload speed less than 5 mbs
Internet connection DSL - location at edge of distance to DSLAM
Internet connection Cable - shared local router is overloaded by some users watching videos online
Remote user Internet connection - slow upload and download connection.
Remote user - insufficient RAM and or too many concurrent applications running.
Remote user local network - high levels of network activity.
Any, several, or all of these can contribute to speed problems.
Remote User rather than IWP is generally faster, sometimes.