Stupid little browser games on Facebook are so ludicrously undemanding that the only possible reason I could see is simply a slow network connection. It's not like they're doing complex 3D rendering like BF3, or reading huge amounts of data from the disk like an MMO.
it could be several reasons.
the ones you can control are browser, flash version, connection speed- well even that is more up to your ISP.
what you cannot control are how the games are "coded" and network congestion.
update your browser and flash then check you connection seep on some place like http://www.speedtest.net/