Minimizing SELECT latency in PostgreSQL

I am evaluating Postgres 9.4 performance on a machine with Intel i7 quad-core 3.6 GHz CPU, 8 GB ram, and 7400 rpm HDD (no RAID) running Linux Mint. The DB schema has the following table:

          Table "public.sensor_readings"
  Column  |           Type           | Modifiers
----------+--------------------------+-----------
 time     | timestamp with time zone | not null
 value    | numeric                  |
 sensor_id| integer                  | not null

Indexes:
      "sensor_readings_pkey" PRIMARY KEY, btree (sensor_id, "time")

This table has 72 million rows and is 35 GB in size (PKEY index is 25 GB). sensor_id ranges from 0 to 5000.
I need to query sensor values for past two weeks:

SELECT 
FROM sensor_readings 
WHERE sensor_id IN (1,3,8,9,12) 
    AND time BETWEEN CURRENT_TIMESTAMP - interval '14 day' AND CURRENT_TIMESTAMP ;

The problem is that the average query execution time is about 5 minutes even after a 2000-execution warm up. This is two orders of magnitude higher that what I want to achieve!

I did not change Postgres default parameters and did no optimization.

Can anyone suggest what can be wrong with my setup or schema? Are there any optimizations that I can do to minimize SELECT execution time? In particular, what is a proper ratio between table (index?) size and RAM?

P.S.

Sample EXPLAIN (ANALYZE, BUFFERS) output:

Bitmap Heap Scan on sensor_readings  (cost=3535.18..405031.07 rows=113904 width=0) (actual time=5190.213..60196.531 rows=103104 loops=1)
  Recheck Cond: ((sensor_id = ANY ('{1509,1504,1503,1500,1502}'::integer[])) AND ("time" >= (('now'::cstring)::date - '14 days'::interval)) AND ("time" <= ('now'::cstring)::date))
  Heap Blocks: exact=47786
  Buffers: shared hit=12 read=48491
  ->  Bitmap Index Scan on sensor_readings_pkey  (cost=0.00..3506.70 rows=113904 width=0) (actual time=5165.932..5165.932 rows=103750 loops=1)
        Index Cond: ((sensor_id = ANY ('{1509,1504,1503,1500,1502}'::integer[])) AND ("time" >= (('now'::cstring)::date - '14 days'::interval)) AND ("time" <= ('now'::cstring)::date))
        Buffers: shared hit=12 read=705

Planning time: 24.887 ms
Execution time: 60205.108 ms