My map is a large, open space dotted with uneven objects - which serve to reduce the visible range.

Would encasing these objects in hint brushes increase the frame rate? The fps is ok as it is (I think; stays above 80), but there's no reason not to improve it further.

With Darkplaces, encasing anything with hint brushes only serves to optimize portals [reduce the number, but not increase the frame rate].

Continue to hide rectangular caulk brushes in irregular detail brushes, this should be an effective compromise between compile time & frame rate.

Relating to this subject, when fog becomes supported, extremely large OPEN terrain maps should be come viable by foghull.

does skytextures or water textures reduce framerates ? when used like hint brushes for covering up the outsides of a map where nobody sepose to be

sky - nope
water - a big yes (they get split up into many small parts for the water moving effect)