Thanks for the help. I talked with the mod creator on the mod page about that. He suggested the same.
To fix this I did the following:
1. Reinstalled the mod with the "Optimized settings" option in the FOMOD installer.
2. I've set the "iMaxDesired=" value at the [Particles] section to "2000" via BethINI
4. I've set "EnableBigRange=", "EnableShadow=" and "EnableNormalMappingShadows=" to "false" in enbseries.ini.
4. I've set "OptimizeSmallRange=" to "true" in enbseries.ini.
This reduced the lag for around 20 - 25 fps when there are a lot of fire sources on screen.