I have news: apparently debug_get_callstack (which GMLive uses to figure out what script is being executed) gets progressively slower the more code you have in scripts that were called - even if that code is commented out. With GMLive now being a massive script, that is an issue.
I filed a bug report (#180892) and will look into mitigating this (likely by splitting the script into smaller scripts).
A workaround is to use live_name, which will then omit calling debug_get_callstack.