The performance degradation occurring when using ULCL

Hello,
Let me ask about the performance degradation occurring when using ULCL.

Are there any known throughput issues when using ULCL?
I’m using iPerf as a measuring tool.
Throughput of DL UDP was about 9 Gbps without ULCL, but dropped to 6 Gbps when ULCL was used.
I’m wondering if ULCL is the cause or not.

If anyone know about this issue, please let me know.
I appreciate for your help.