Hello,
Does this happen even when adding new highlight for a new page?
Or only when adding new highlight for existing page that already have many other words to highlight?

In general, the highlighting is slower when you have:
- many words to highlight on a single page
- if you are using regular expressions (RegExp)
- if the page where it runs is complex/huge, with a lot of elements (like Facebook / Reddit)

I plan to look into some performance optimization in the new release (in a few weeks).

You can also contact me here:
admin@fastaddons.com

Best Regards,
Juraj Mäsiar