iperf3 single-stream low bandwidth with small message sizes (1KB, 1500B, 2000B, etc.)

# Context

* Version of iperf3: iperf 3.5

* Hardware:
Nvidia ConnectX-6 Dx 100GbE
Intel XXV710 25GbE

* Operating system (and distribution, if any):
Red Hat Enterprise Linux Server release 7.5 (Maipo) - kernel 5.10.0-rc4
CentOS Linux release 8.1.1911 (Core) - kernel 5.2.9

# Bug Report
We are observing low performance with iperf3 while sending single-stream traffic with small message sizes.
Other benchmarks reports significant better results for same scenario, we assume it's Iperf3 that's limiting the performance we observe.

For example:
Running `iperf3 -c fd12:fd07::200:105 -t 10 -l1500B` we see ~7.9Gbps.

Running the same test with 1 stream and 1500B with **iperf2** we see 23Gbps which is what we would expect with current system.
cmd: `iperf -c fd12:fd07::200:105 -V -l1500 -P1`

* Expected Behavior

Iperrf3 should be able to achieve at least the same performance reported by iperf2 for a single-stream.

* Actual Behavior

Iperf3 small message size performance is low. Iperf3 is the limiting factor.

* Steps to Reproduce

Send traffic with message size 1500B (On a high performance NIC over 10/25Gbps).

* Possible Solution

I've noted iperf2 suffer a similiar issue when running with '-i' flag, might be related to the reporting/results gathering flow?