kafka producer takes much more time when writing to more partitions

we have 5 kafka brokers, replication factor 3, and 2 topics - T1 and T2.

T1 topic has 100 partitions, T2 has 6.

when writing with a kafka producer (using KafkaProducer.send()) to both T1 and T2, the time it takes to the producer.send() is ~1 sec.

But after increasing the number of partitions in T2 from 6 to 100, the total time of the send() increased to 20 sec.

Any idea why?