By default during the recording of a Visual Test, SilkTest Workbench will take a screenshot when a user performs an action, this pauses the browser until the screenshot is taken and all properties of controls are captured. Once this is complete, the browser is released to continue. It is this process which causes the slowness of recording and playing back of visual tests. The proposed workaround is to turn off Control Capture and Screen capture from the Tools -> Options menu from within SilkTest Workbench.  This information is present in the SilkTest 13.5 release notes on page 6. 

 The reason that these options must be turned off for Google Chrome is that prelimenary support includes the recording of locators and playback only.   Visual Tests will need to be recorded using the  Identify Object tool.  This tool will permit you to click on an item that you wish to interact with and select an action that you wish to perform on the item you have clicked on.  This recorded interacton can then pasted to the Visual Test as a test step. 

Currently SilkTest 13.5 currently supports the following Google Chrome and Mozilla Firefox versions:-
Google Chrome 19
Google Chrome 20
Google Chrome 21

Mozilla Firefox 12
Mozilla Firefox 13
Mozilla Firefox 14