Explain analyse for slow query

finder_development=> explain analyse SELECT *, (ACOS(least(1,COS(0.659296894483735)*COS(-2.13662186884198)*COS(RADIANS(groups.lat))*COS(RADIANS(groups.lng))+ COS(0.659296894483735)*SIN(-2.13662186884198)*COS(RADIANS(groups.lat))*SIN(RADIANS(groups.lng))+ SIN(0.659296894483735)*SIN(RADIANS(groups.lat))))*6376.77271) AS distance FROM "groups" WHERE (defunct = false AND lat is not null and lng is not null and full_address is not null and full_address != '') ORDER BY (ACOS(least(1,COS(0.659296894483735)*COS(-2.13662186884198)*COS(RADIANS(groups.lat))*COS(RADIANS(groups.lng))+ COS(0.659296894483735)*SIN(-2.13662186884198)*COS(RADIANS(groups.lat))*SIN(RADIANS(groups.lng))+ SIN(0.659296894483735)*SIN(RADIANS(groups.lat))))*6376.77271) ASC, num_members DESC LIMIT 30;;
                                                                                                                                                                                                                   QUERY PLAN                                                                                                                                                                                                                    
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 Limit  (cost=771.03..771.10 rows=30 width=363) (actual time=35.406..35.413 rows=30 loops=1)
   ->  Sort  (cost=771.03..783.24 rows=4885 width=363) (actual time=35.404..35.405 rows=30 loops=1)
         Sort Key: ((acos(LEAST(1::double precision, (((((-0.423756013910457)::double precision * cos(radians((lat)::double precision))) * cos(radians((lng)::double precision))) + (((-0.667232757379067)::double precision * cos(radians((lat)::double precision))) * sin(radians((lng)::double precision)))) + (0.612561252574016::double precision * sin(radians((lat)::double precision)))))) * 6376.77271::double precision)), num_members
         Sort Method:  top-N heapsort  Memory: 40kB
         ->  Seq Scan on groups  (cost=0.00..626.75 rows=4885 width=363) (actual time=0.038..26.495 rows=5126 loops=1)
               Filter: ((NOT defunct) AND (lat IS NOT NULL) AND (lng IS NOT NULL) AND (full_address IS NOT NULL) AND ((full_address)::text <> ''::text))
 Total runtime: 35.489 ms
(7 rows)