ReadyNAS RN2120 painfully slow.

I apologize if this has been covered but could not find an answer that helps me.

I work for a manufacturing company and we use an RN2120 to save normal PDFs, Microsoft office files, pictures, and files for SolidWorks. The NAS has always been a little bit slow when opening or saving SolidWorks Files but within the last few weeks it has become slow to open any file. Pictures rarely load anymore on PCs, even thumbnails usually do not load, but it isn't consistent as one or more computers will access the pictures like normal (this can change within a matter of minutes). 

Once I learned that everything had been loading slow I started looking into the settings of the NAS and nothing looks out of the ordinary, all the drives are showing good, tried the alternate network card, etc. The other kicker to the problem is that one PC can be slow and none of the others but then that PC can start being slow and the others will too or they may not... Basically no consistency to anything. We do have another Dell Precision 5810 that is used to host a QuickBooks company file and really nothing else, as a test I had direct copy of one of SolidWorks files copied to that computer and recorded the time it took to open it on the same PC, from the Nas it was around 5 minutes, from the Dell it was 20 seconds, and directly from the hard drive on the PC it was 10 seconds so I am ruling out any network issues.

Anyone have any ideas? I am sure I am missing something simple (hopefully) but if I cannot fix this soon I will be forced to replace this NAS with a different one and likely a different brand as it has not had the desired speed since the beginning. I am going to take the NAS apart this weekend to test each drive individually with a 3rd party program as well try to test anything else inside although I am not very hopeful I will find anything wrong. Also to note I have defragmented the drive and the antivirus is up to date, also scanned with Malware Bytes in hopes someone dropped an infected file in there. Firmware is 6.4.0; it was updated to 6.4.2 around the time things really slowed down so I reverted it back which helped with speed issues a lot but still not were it should be. The system is 4x 2TB drives running in Raid 5 configuration, there is 5.34TB of overall savable space with 4.72TB free. If I missed any need information please let me know and I will post it ASAP. 

Thank you in advance for any help on.