C2960X iperf3 limited by srr-queue bandwidth?

[chrismes]

We've a WS-C2960X-48LPS-L with 15.2(7)E9.
After we've got new provider-line we tried to test upload/download with iperf3 on a windows-pc.
Download is fine with 20Mbit/s, but upload is only 2Mbit/s. It should be the same speed like download.
Unfortunately the routers are not controlled by us, we only support the switch.
But provider told us, he is not traffic-shaping or using QoS.
On our switch there is srr-queue bandwidth shape and share on all our switchports. I think it is because of the VoIP-phones.
So I am wondering if this srr-queue bandwidth could have influence on iperf3?
But I've already disabled it on the uplink ports and on the port where the iperf-pc is connected.
I also tried to disable mls qos globally.
But it is always the same result, just 2Mbit/s upload.
Any suggestions?
Thanks.

[Joseph W. Doherty]

"So I am wondering if this srr-queue bandwidth could have influence on iperf3?"

Possibly.

The 2960 series doesn't have lots of buffer RAM and QoS, on that switch, can often compound the effect of the lack of buffer RAM.

Interfaces and/or QoS stats showing drops?

Another possibility, not mutually exclusive with the above, if you egress bursts above 20 Mbps, your ISP might be dropping traffic too, and if doing enough of that, throughput can be very much reduced.

Iperf stats show packet loss?