Investigations:
Check switch port statistics for drops and queue overflows on the uplink port. Run iperf pairwise first to confirm individual links are healthy, then with all streams simultaneously to reproduce the problem. Capture with tcpdump and look for TCP retransmissions and shrinking window sizes indicating loss.
Possible causes:
Interconnect. Multiple streams share a single uplink whose bandwidth is less than their aggregate — the classic fan-in problem. Four hosts at 1 Gbps into a 1 Gb