The replacements need some server power/time. The settings do indeed have effect.

Here a few pointers to make them as light as possible:

Normal search vs. Regular expressions
Regular expressions are heavier than normal searches (especially when using references).
So use normal searches when you can. You can also use lists to search (comma separated)

Area setting
The 'Enable in area' settings 'Body' and 'Everywhere' are the fastest.
When you set this to 'Articles', the replacement will be done on each article. So it will be done multiple times on blog pages.

Enable in Tags
Keep setting 'Enable in Tags' to 'yes' if you can.
If you set this to 'No' or 'Only', ReReplacer has to find the parts in which to replace and where not.