Slow Consumers on Kafka - apache-kafka

Question

I use Kafka to maintain a Python service which should be working on parallel to handle the slow API requests for each message efficiently.
I used multiprocessing module on Python and kafka-python for the consumers.
ZooKeeper and Kafka 2.11 runs on the same Ubuntu server with mostly defult configurations.
The topic is auto-created with another kafka-python producer and set to have 10 partitions in order to use up to 10 consumers at the same time.
When I check, I see that the queue is really long, so the producer sends so many requests:
$ bin/kafka-run-class.sh kafka.tools.GetOffsetShell --broker-list localhost:9092 --topic usrReq --time -1
usrReq:8:1157
usrReq:2:1185
usrReq:5:1167
usrReq:4:1115
usrReq:7:1164
usrReq:10:1150
usrReq:1:1149
usrReq:9:1138
usrReq:3:1186
usrReq:6:1220
usrReq:0:6264
However; although working 10 cores in parallel, consumers take very long time (117 seconds on a sample log below) to get the next message from the queue.
thread 7, consumer: 117.485 sec
api1:0.412 sec
api2:0.752 sec
db_insert:0.132 sec
This is how each Process creates its own consumer, fetches messages and runs the analysis on the code:
consumer = KafkaConsumer(group_id='my-group',
bootstrap_servers='localhost',
value_deserializer=lambda m: json.loads(m.decode('ascii'))
consumer.subscribe(topics='usrReq')
while True:
msg = next(consumer).value['id']
method(msg)
Where could be the problem in this setup?