As @melvinius said, an update can be the cause of the FPS drop. For example, I just updated Water for ENB. In the FOMOD installer, by mistake, I chose the default 4K resolution. My frame rate in certain areas of the map tanked to 15 to 20 FPS. I reinstalled the update with the 2K option (which I should have done if I was paying attention in the first place) and my FPS returned to normal. Take a look at any mods that deal with things like fog, light, water, or other environmental elements that give the GPU a "workout" and see if you updated anything.