Very slow query despite index being used
There are two indices on (insert_time) and (insert_time DESC). B-tree indices can be scanned backwards at practically the same speed. And insert_time is NOT NULL, so there is no point whatsoever. Drop one of those in any case.

I made some assumptions where info is missing:


Current Postgres 12.
You are free to redesign the table and lock the table exclusively for some time.
The table is basically "append only".
New rows are added in order of insert_time.
The current table is physically out of order and/or bloated (leading to the index scan in your query plan instead of bitmap index scan)


I would rewrite the table like this:

BEGIN;
-- SET maintenance_work_mem = ???   -- set as high as you can afford temporarily

-- drop idx first to free space
DROP INDEX public.idxgin;     -- acquires exclusive lock on the table
DROP INDEX public.tv_smartdev_insert__0ae03a_idx;
DROP INDEX public.tv_smartdevicemeasurement_modbus_smart_device_id_62c12ed0;
-- DROP INDEX public.tv_smartdev_insert__1f0611_idx; -- might help SELECT, drop later

ALTER TABLE public.tv_smartdevicemeasurement_modbus ALTER measurement_id DROP DEFAULT;
DROP SEQUENCE public.tv_smartdevicemeasurement_modbus_measurement_id_seq;  -- drop owned sequence
ALTER TABLE public.tv_smartdevicemeasurement_modbus
         RENAME TO tv_smartdevicemeasurement_modbus_old;    -- free org. name

CREATE TABLE public.tv_smartdevicemeasurement_modbus (
   measurement_id   serial PRIMARY KEY    -- consider IDENTITY column instead, see below
 , smart_device_id  integer NOT NULL      -- reordering saves 4-8 bytes alignment padding per row
 , insert_time      timestamp with time zone NOT NULL
 , data             jsonb NOT NULL
 , CONSTRAINT tv_smartdevicemeasur_smart_device_id_62c12ed0_fk_tv_smartd
      FOREIGN KEY (smart_device_id)
      REFERENCES public.tv_smartdevice_modbus (device_id) DEFERRABLE INITIALLY DEFERRED  -- WHY deferrable / deferred? see below
);

INSERT INTO public.tv_smartdevicemeasurement_modbus
      (measurement_id, smart_device_id, insert_time, data)
SELECT measurement_id, smart_device_id, insert_time, data
FROM   public.tv_smartdevicemeasurement_modbus_old
ORDER  BY insert_time DESC;   -- CLUSTER while rewriting

DROP TABLE tv_smartdevicemeasurement_modbus_old;

CREATE INDEX tv_smartdev_insert__1f0611_idx ON public.tv_smartdevicemeasurement_modbus (insert_time DESC);
-- CREATE INDEX tv_smartdev_insert__0ae03a_idx ON public.tv_smartdevicemeasurement_modbus (insert_time);  -- nope!

CREATE INDEX idxgin ON public.tv_smartdevicemeasurement_modbus USING gin (data);  -- ?
CREATE INDEX tv_smartdevicemeasurement_modbus_smart_device_id_62c12ed0 ON public.tv_smartdevicemeasurement_modbus (smart_device_id); -- ?

COMMIT;

VACUUM ANALYZE public.tv_smartdevicemeasurement_modbus;


This rewrites the table saving some space (which also helps performance). Most importantly, it clusters the table according to your main index - and removes all possible bloat while being at it. This should help locality of data and make Postgres read fewer data pages. Unless your data column is big, you should see bitmap index scan in the query plan now. And if data is small, consider a covering index  to get index-only scans. See:


Does a query with a primary key and foreign keys run faster than a query with just primary keys?


How does the changed column order save space?


Calculating and saving space in PostgreSQL


DEFERRABLE INITIALLY DEFERRED? This is rarely necessary. And cheaper without. See:


SET CONSTRAINTS ALL DEFERRED not working as expected


serial vs. IDENTITY? See:


How to change a table ID from serial to identity?


Then query like this:

SELECT data
FROM   tv_smartdevicemeasurement_modbus
WHERE  insert_time &gt;= '2019-08-01'  -- included
AND    insert_time &lt;  '2019-09-25'; -- excluded


Be aware that these date literals are interpreted according to your local time zone setting. Consider true timestamptz input to be unambiguous. See:


Ignoring time zones altogether in Rails and PostgreSQL


These two indices are orthogonal to the query at hand: idxgin and tv_smartdevicemeasurement_modbus_smart_device_id_62c12ed0. Drop unless needed for unrelated stuff.