Hard to say, do other non-casino programs do the same? If so you have a computer problem or an older system.
Online Casinos really suck up alot of the system resources.
I noticed sometimes the Microgaming Casinos don’t have any sound when playing slot games. Other times they’re fine.
In any event you should play a casino that has servers located nearest you for best results. Plus it also depends on how many others are online playing.
Look at this site under online casinos, it lists the location of some of the game servers for some of the Microgaming Casinos. There is also a section of free flash games on another page, try them and see if they work, they’ll eat up less system resources.
Unless you’re already playing flash based casinos and that’s the problem, then you can download a full casino and play in guest mode and see if that works better.