As mentioned by others, this is not playing well with the recent WP update - even though it's not actually being used.
If it's installed and ACTIVATED it causes the issue of slowing the entire site to near full stop (though most pages pretty much time out).
If it's installed and DEACTIVATED, everything works fine!
I've confirmed this with about a dozen sites.
This is a very useful plugin - especially when first building a site, but also when longer maintenance tasks are needed.
Thanks to the Developer for his great work & continued support of WP.