Reducing execution time of PostgreSQL stored function - database

Question

So in Postgres I created a function that creates a table vehicletracks_tracks and fills in with information calculated from original table. Original table includes the following atributes: id bigint NOT NULL DEFAULT, session character varying(32), client_id character varying(32) NOT NULL, vehicle_type smallint NOT NULL, geolocation geography(Geometry,4326) NOT NULL and "timestamp" timestamp without time zone NOT NULL.
In the function I calculate tracks and borders between them by observing time and velocity between two consecutive rows in original table. A fter creating tracks I calculate distance and duration of every track.
The function takes around 6 minutes and 30 seconds and then the output table is created and filled with rows. The original table has around 5 million rows. When you look at the procedure is there any possibility to reduce its execution time?
This is my function:
CREATE OR REPLACE FUNCTION vehicletracks()
RETURNS void AS
$BODY$
DECLARE
vrstica1 RECORD;
vrstica2 RECORD;
track_number INT;
BEGIN
SET TIME ZONE 'UTC';
DROP TABLE IF EXISTS vehicletracks_tracks;
CREATE TABLE vehicletracks_tracks (
session character varying(32),
client_id character varying(32) NOT NULL,
vehicle_type smallint NOT NULL,
track_number SERIAL PRIMARY KEY,
track_distance DOUBLE PRECISION,
track_duration INTERVAL,
track_start TIMESTAMP WITH TIME ZONE,
track_stop TIMESTAMP WITH TIME ZONE
);
DROP VIEW IF EXISTS group_by_client_session_vehicle;
CREATE VIEW group_by_client_session_vehicle AS
SELECT DISTINCT client_id, session, vehicle_type
FROM roaduserspositions
GROUP BY client_id, session, vehicle_type
ORDER BY session, client_id, vehicle_type;
FOR vrstica1 IN (SELECT * FROM group_by_client_session_vehicle)
LOOP
DROP VIEW IF EXISTS velocity_difference_view;
DROP TABLE IF EXISTS difference_table;
CREATE TABLE difference_table AS
SELECT id, session, client_id, vehicle_type, geolocation, timestamp, time_difference, distance_difference
FROM (SELECT t.*, t.timestamp - LAG(t.timestamp) OVER (ORDER BY t.session, t.client_id, t.vehicle_type, t.timestamp ASC) AS time_difference,
ST_Distance(geolocation, LAG(geolocation) OVER (ORDER BY t.session, t.client_id, t.vehicle_type, t.timestamp ASC)) AS distance_difference
FROM (SELECT *
FROM timonwww_roaduserspositions
WHERE client_id = vrstica1.client_id AND session = vrstica1.session AND vehicle_type = vrstica1.vehicle_type) AS t) AS tab
ORDER BY session, client_id, vehicle_type, timestamp;
CREATE VIEW velocity_difference_view AS (
SELECT *, distance_difference / EXTRACT(EPOCH FROM time_difference) AS velocity
FROM difference_table
WHERE EXTRACT(EPOCH FROM time_difference) > 0
UNION
SELECT *, 0 AS velocity
FROM difference_table
WHERE EXTRACT(EPOCH FROM time_difference) = 0
ORDER BY session, client_id, vehicle_type, timestamp);
DROP TABLE IF EXISTS new_time_difference;
CREATE TABLE new_time_difference (
track_id INTEGER,
LIKE velocity_difference_view INCLUDING ALL);
track_number := 1;
INSERT INTO new_time_difference(track_id, id, session, client_id, vehicle_type, geolocation, timestamp, time_difference)
SELECT
CASE
WHEN EXTRACT(EPOCH FROM time_difference) < 300 AND velocity < 121 THEN track_number
ELSE track_number + 1
END AS track_id, id, session, client_id, vehicle_type, geolocation, timestamp, time_difference
FROM velocity_difference_view OFFSET 1;
track_number := 1;
FOR vrstica2 IN (SELECT * FROM velocity_difference_view OFFSET 1)
LOOP
IF EXTRACT(EPOCH FROM vrstica2.time_difference) < 300 AND vrstica2.velocity < 121 THEN
INSERT INTO new_time_difference VALUES(track_number, vrstica2.id, vrstica2.session, vrstica2.client_id, vrstica2.vehicle_type, vrstica2.geolocation, vrstica2.timestamp, vrstica2.time_difference);
ELSE
track_number := track_number + 1;
END IF;
END LOOP;
DROP TABLE IF EXISTS geolocation_difference_table;
CREATE TABLE geolocation_difference_table AS
SELECT *
FROM (SELECT *, ST_Distance(geolocation, LAG(geolocation) over (ORDER BY timestamp ASC)) as geolocation_difference
FROM new_time_difference
WHERE track_id IN (SELECT * FROM generate_series((SELECT MIN(new_time_difference.track_id) FROM new_time_difference), (SELECT MAX(track_id) FROM new_time_difference)) num)) AS tab;
DROP TABLE IF EXISTS track_distance_table;
CREATE TABLE track_distance_table AS
SELECT track_id, SUM(geolocation_difference) AS track_distance, SUM(time_difference) AS track_duration, MIN(timestamp) AS track_start, MAX(timestamp) AS track_stop
FROM geolocation_difference_table
GROUP BY track_id
ORDER BY track_id ASC;
INSERT INTO vehicletracks_tracks(session, client_id, vehicle_type, track_distance, track_duration, track_start, track_stop)
SELECT vrstica1.session, vrstica1.client_id, vrstica1.vehicle_type, track_distance, track_duration, track_start, track_stop
FROM (SELECT * FROM track_distance_table WHERE track_distance > 0 AND track_distance IS NOT NULL) AS t;
END LOOP;
DROP TABLE IF EXISTS vehicletracks_binding;
CREATE TABLE vehicletracks_binding AS
SELECT id, track_id
FROM geolocation_difference_table
ORDER BY track_id ASC;
DROP VIEW IF EXISTS group_by_client_session_vehicle;
DROP VIEW IF EXISTS velocity_difference_view;
DROP TABLE IF EXISTS difference_table;
DROP TABLE IF EXISTS new_time_difference;
DROP TABLE IF EXISTS geolocation_difference_table;
DROP TABLE IF EXISTS track_distance_table;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;