Hey there Ungnome,
That sounds a bit odd that the FPS is stuck around 60. We’ve seen Razer doing this with some other games. It is possible it’s affecting Diablo 3 also. Most report that disabling the Razer Chroma API in Razer Synapse works. That’s done by going to Connect → Apps → Turn off Diablo III.exe
If you don’t see that or see no difference with it off, uninstalling Razer and restarting the system can also work to troubleshoot.
Let us know if that helps out. Cheers!