Network Drive attached as SCSI Dropping the W/R speed on bigger files

I'm having Network disk attached a SCSI disk

When copying smaller files like 700MB or even 1 GB, speed is high , from 90MB/s to 120 MB, so I'm capable to benefit from 1Gbit network. But if files is around 4 GB, the speed is dropping in 10 Seconds and goes to 15-8 MB/s , I understand its getting capped, So I'm wondering, shall I look in to the router, some sort of burst feature, or windows has some sort of capping too?

You have caches at both sides so while writes are write-buffered you get wire speed, eventually performance goes down.

https://blogs.technet.microsoft.com/josebda/2014/08/18/using-file-copy-to-measure-storage-performance-why-its-not-a-good-idea-and-what-you-should-do-instead/

https://sqlperformance.com/2015/08/io-subsystem/diskspd-test-storage

Make sure you disable cache when testing!

Parameter Description

-h Disable software caching at the operating system level and hardware write caching