Hello,
Thank you for the additional information.
I was able to reproduce the reduced speed that you experienced and I think I have discovered the cause of it, Can you please try this modified build and let me know if it resolves the problem for you. If not can you please post the speed in FlashFXP vs the other client you're testing.
Download ffxp5.0.0.3805-r3.zip
Unzip the flashfxp.exe into your FlashFXP program folder overwriting the existing flashfxp.exe file.