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The description in the official bug report is very clear: this is not a bug.

This is what is happening:

1. With a queue size of 1, and a buffer size smaller than your message size, the first message is processed with little delay, but the second message gets stuck in limbo: 64k of the message is in the subscriber's buffer, and the remainder is stuck in either the OS buffer or the publisher's buffer.

2. While the second message is stuck halfway transmitted in the publisher's queue, it may or may not drop subsequent message, depending on the publisher queue size.

3. Once your subscriber is done with the first message, rospy is free to finish the receive of the second message, and then your subscriber is triggered. The following message (3 if the publisher has not dropped any messages, or a later message if it is dropping messages), now becomes stuck halfway.

4. (2) and (3) repeat, and the delay should eventually settle out to the publisher queue size times your processing time.

Increasing the subscriber buffer size allows the subscriber to receive a complete message, instead of having it stuck halfway, and then the message can be put into the pending message queue and dealt with according to the queue size.