Alan
You should use the 'SSL Settings' section which is in the Admin->SIte Settings page. Mark your site as 'SSL Enforced' and 'SSL Enabled'. Then, on the pages which are to be secure only, find the 'Secure' checkbox under 'Other Settings' in the Page Settings.
Note this is standard DNN behaviour and nothing to do with the Friendly Url Provider.
As for performance, the lookups are done with a typed dictionary collection, which is supposed to not be affected by collection size. The only performance hit will be done when the collection is built, which is done at system startup or when the cached collection expires due to a new page being added, or a page name being changed, or adding in a new redirect. You can read your event log to see how often the page index is being rebuilt.
-Bruce