The image rollovers are CSS3 transistions/animations and we have set a default value for them. If you would like to change this, follow the step below.
Edit css/all.css, find and change 0.3s to a lower value to speed up the animation, or a higher value to slow it down.
Please note this is a customization that is outside of what we can provide for theme support. If you perform the customization, you will need to maintain the changes for future updates and also be able to troubleshoot any issues that arise. Thank you.