Hmm could it be a rogue while loop somewhere in the custom commands? It's also possible a custom command is not using the "appIsPlayingAndExcecute" boolean, which can be used to avoid running code when text is simply viewed in the editor. For example, if that variable is removed from an "add" method, it will add to a value every frame the text is viewed in-editor.
If those two aren't an option, please send me an email with your invoice no., I have a new build of Fleece that's 99% ready for release that fixed/adjusted a ton of small things, so it's possible this could be a bug I've fixed already. Maybe.