Read and Write Performance drops over uptime

[NASnewbi916]

So i have been having this issue for a long time and it isn't really noticeable unless i have to do large file reads or writes of video files. Here is the issue...

If i reboot the TrueNAS Core server the read and write speeds are very high i can get about 1.3Gb/s however, after the system has been running for example right now its running for over 5 days, the speed drops to maybe 50Mb/s read. I have attached an image with system info it is sporting dual Xeon E5-2680 v2 CPUs in a Dell R720xd with 512GB of ram.

Please advice as to what might be an issue