What will affect linux udp throughput or sending rate?

Question

I am testing an linux app(ubuntu, udp socket to send data, 10Gbps NIC).This app will set the udp packet interval from big to small, e.g, from 12000 us to 1.2us. The udp packet sending rate is expected to be from 1Mbps to 10Gbps(1500Bytes packet).

However it is only an ideal solution. In fact, when the rate is above 200Mbps, the difference between the actual sending rate(computed from wireshark) and the expected sending rate began to grow(from 10% to 100%...300%).

I found that the actual smallest packet interval only could be 5-6us, which is bigger than the smallest expected value-1.2us. And I thought if I increase the udp packet size, e.g from 1500Bytes to 9000Bytes, the smallest 6us packet interval can also get 10Gbps sending rate.

But in fact the smallest packet interval inceases from 5us to 13-14us, when I increase udp packet size from 1500Bytes to 9000Bytes.

So my question is what will affect the actual packet interval(computed from wireshark)? How can I ensure the actual packet interval is as consistent as possible with the expected one.

Thank you.