INDEX Performance Issue

Here is the structure of my database tables

CREATE TABLE data
(
  id bigserial NOT NULL,
  datasetid integer NOT NULL,
  readingdatetime timestamp without time zone NOT NULL,
  depth double precision NOT NULL,
  readingdatetime2 timestamp without time zone,
  depth2 double precision,
  value double precision NOT NULL,
  uploaddatetime timestamp without time zone,
  description character varying(255),
  point geometry,
  point2 geometry,
  CONSTRAINT "DATAPRIMARYKEY" PRIMARY KEY (id ),
  CONSTRAINT enforce_dims_point CHECK (st_ndims(point) = 2),
  CONSTRAINT enforce_dims_point2 CHECK (st_ndims(point2) = 2),
  CONSTRAINT enforce_geotype_point CHECK (geometrytype(point) = 'POINT'::text OR point IS NULL),
  CONSTRAINT enforce_geotype_point2 CHECK (geometrytype(point2) = 'POINT'::text OR point2 IS NULL),
  CONSTRAINT enforce_srid_point CHECK (st_srid(point) = 4326),
  CONSTRAINT enforce_srid_point2 CHECK (st_srid(point2) = 4326)
);

CREATE INDEX data_datasetid_index ON data USING btree (datasetid );
CREATE INDEX data_point_index ON data USING gist (point );
CREATE INDEX "data_readingDatetime_index" ON data USING btree (readingdatetime );
ALTER TABLE data CLUSTER ON "data_readingDatetime_index";

CREATE TABLE area
(
  id serial NOT NULL,
  "areaCode" character varying(10) NOT NULL,
  country character varying(250) NOT NULL,
  "polysetID" integer NOT NULL,
  polygon geometry,
  CONSTRAINT area_primary_key PRIMARY KEY (id ),
  CONSTRAINT polyset_foreign_key FOREIGN KEY ("polysetID")
      REFERENCES polyset (id) MATCH SIMPLE
      ON UPDATE CASCADE ON DELETE CASCADE,
  CONSTRAINT enforce_dims_area CHECK (st_ndims(polygon) = 2),
  CONSTRAINT enforce_geotype_area CHECK (geometrytype(polygon) = 'POLYGON'::text OR polygon IS NULL),
  CONSTRAINT enforce_srid_area CHECK (st_srid(polygon) = 4326)
);

CREATE INDEX area_polygon_index ON area USING gist (polygon );
CREATE INDEX "area_polysetID_index" ON area USING btree ("polysetID" );
ALTER TABLE area CLUSTER ON "area_polysetID_index";

CREATE TABLE data_area
(
  data_id integer NOT NULL,
  area_id integer NOT NULL,
  CONSTRAINT data_area_pkey PRIMARY KEY (data_id , area_id ),
  CONSTRAINT data_area_area_id_fk FOREIGN KEY (area_id)
      REFERENCES area (id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION,
  CONSTRAINT data_area_data_id_fk FOREIGN KEY (data_id)
      REFERENCES data (id) MATCH SIMPLE
      ON UPDATE CASCADE ON DELETE CASCADE
);