Help bad results with pgbench

Good morning everyone.

I'm having the following problem with pgbench and test again:

I create a database with a scale of 200 and execute a test (read only) with
40 users and this is the result:

pgbench -U postgres -p 5433 -j 10 -c 40 -T 30 -h 127.0.0.1 -S pgbench

scaling factor: 200

query mode: simple

number of clients: 40

number of threads: 10

duration: 30 s

number of transactions actually processed: 2682306

latency average = 0.446 ms

latency stddev = 0.303 ms

tps = 89329.960851 (including connections establishing)

tps = 89348.642359 (excluding connections establishing)

Then restart the server and when I run the test again the result is really
very bad and the IO starts to climb (here the weird thing is that I'm not
writing just reading)

scaling factor: 200

query mode: simple

number of clients: 40

number of threads: 10

duration: 30 s

number of transactions actually processed: 6933

latency average = 174.032 ms

tps = 229.842173 (including connections establishing)

tps = 229.870409 (excluding connections establishing)

Can someone give me some suggestions so I can determine what's going on?

PostgreSQL 9.6.2.

Thank you very much for your time.

Regards.