Naddo the Anointed and FOA final campaign's boss' certain attacks cause bad lag/fps drops

While fighting Naddo the Anointed and Beronath Corruptor of Asterkarn (especially its second phase), the FPS drops significantly during certain attacks.

These attacks seem to be:

Naddo - attack kinda similar to Hagarrad rune waves of spikes;
Beronath - [SkillSecondary_AttackRadiusLightning] Chaos Lightning field.

I play on ultra settings at 4k resolution and MSAA 2x, but here I had to lower shadows and particles to check if it helped, and it still lagged. I dont use deferred rendering as I dont like FXAA blur.

This affects gameplay as Beronath’s [Skill_AttackLineFan] can oneshot 4500 armor 20 phys res 86 percent chaos res fire blade arc commando with 21/22 menhir bulwark and defensive devotions from 18000 HP to zero in ascendant mode if it shotguns and lag kinda makes it hard to judge the timing to dodge.

My setup (Windows 10, no applications open except Grim Dawn):
Ryzen 5800X3D CPU
32 GB DDR4 RAM
GPU:NvidiaRTX 5060Ti 16GB
2 TB ADATA SSD NVME
Gsync compatible monitor

Attaching screenshot of my settings

One attack in particular, I believe it’s this one. He sends red ripples along the ground which consistently lowers fps to single digits.
Capt33ure