An i5 4590 would defiantly hold you back in Assassin's Creed Odyssey. Run MSI Afterburner and setup up the OSD to show the following.
CPU Usage and Temp
GPU Usage and Temp
Memory usage
FPS Average and Latency
At 1080p High/Very High settings you're going to see the CPU Hit 100% load while the GPU will hover around 50-60%. AC: Odyssey is more CPU dependent than it should be since it is very much unoptimized so you need the ability to overclock as high as possible. Try these settings and see if you notice a difference.
In Task Manager set Odysseys priority to high.
"Disable Fullscreen Optimization"
Go to your ACOD Install directory and right-click the ACOdyssey.exe file.
Select the Compatibility Tab and Check "Disable Fullscreen...