How to solve safari slow script error

This error prompt is easily remedied, however, you'll need to access the Safari Debug Menu to do so. If the Debug Menu is activated, click on it in the Menu Bar and uncheck "enable runaway JavaScript timer. That ought to solve this problem.

If the Debug menu is not activated, follow these instructions:

Quit Safari
Open Terminal.app in your Utilities folder,
When the Terminal window opens, copy & paste the following command into the window:

defaults write com.apple.Safari IncludeDebugMenu 1

Click on the return key, you may be prompted for an admin password. If so, enter it.
Press "return".
When you see your Mac's name, it's done,
Quit Terminal
Restart Safari and go to the Debug Menu, following the suggestion above.