Huge performance drop in last phase of Sirus fight

This phase is the one in which :

That thing hovering above creates that gigantic maze, 3 levels and you exit it at the edge of the arena, well it fails to render or renders too slowly, so I never know when/where to escape.

Thing hovering above starts his red-coloured beam in counter-clockwise direction, Sirus starts shooting his ailment beam - this causes my system to lag, that dynamic resolution thing kicks in and frame rate drops close to 0.

Now, none of this Dynamic resolution thing should be happening, because there's just me, Sirus and his minion/thing/whatever the heck it is.

Also I play minions, so there are 5 spectres, 10 zombies, 8-10 skeletons and 1 stone golem.


Editing with some system details :

Type : laptop
OS : Windows 10 Home - 10.0.17134(build I am not sure about)
RAM : 8GB
processor : i5-4210U
Graphics card : Geforce GTX 850M
connection : 20mbps down/512kbps up

I can confirm that (have all same problem as your).
1) Maze - slow load [when 1st time he use].
2) Red beam in counter-clockwise - FPS drop near 0 [when 1st time he use], this is force to death if bad standing location .
3) Ailment beam - FPS drop near 0 [Every time he use], this is force to death.

OS : Windows 7
RAM : 16GB
processor : i5-4440
Graphics card : Geforce GTX 1050Ti
connection : Cable 20mbps down/1mbps up

same exact problems here, would be getting deathless kills if not for completely locking up in the last phase.

OS : Windows 10
RAM : 16GB ddr4
processor : Ryzen R7 1700@3.7
Graphics card : Geforce GTX 1070
connection : verizon Fios 75/75

I don't know if it's the right place to write and I apologize for my bad English, but I have the exact same problem in this encounter. In the last phase when he cast his beam and other skills, the game just freezes or lags very badly for several seconds. And I've discovered a some weird thing about it (might be just a coincidence). I've had this issue several weeks ago, when I played on Moscow gateway (because it's the closest to me), then because the game was unplayable during peak times (latency over 2k and it automatically connected to Frankfurt for some reason), I've switched to Amsterdam and London gateways and lag spikes during this encounter were bearable. Only when he says "Die!" there were very annoying freezes for 1 second or so. But after the recent patch I've switched back to the Moscow gateway and while everything worked fine and latency was around 15ms, I've had very bad lag spikes and freezes again during Sirus fight. I haven't recorded a video and took a screenshot too late, but you can see what happens when he cast anything
Only due to very good gear and knowledge of the fight I haven't died this time, but back then these lags cost me a kill 2 times.

Specs:
CPU: i7 940
GPU: GeForce GTX 980ti
18GB RAM
OS: Windows 10 Pro 1909 (Build 18363.535)
connection: cable 100mbps

Also having issues during the last phase of the fight. I am also running minions. 3 Specters, 2 Skitter bots, 14 Zombies, 13+ Skeletons, 11 Wraiths, 1 Carrion Golem. (Minions also taunt). Seems to kick in when he does the rotating beam attack. Happened on his second use of it.

OS: Windows 10
GPU: GeForce GTX 750ti
RAM: 8 GB
CPU: AMD Phenom II 1090T

Necro with minions. Phase one and two absolutely no problems. Phase 3: rotating beam, death beam -> 0 fps, lost all portals and nearly a whole level. Extremely frustrating, especially as it will take another 50 maps to spawn him.

OS: Windows 10
GPU: GeForce GTX 960
RAM: 8 GB
CPU: AMD fx 4300

same situation here
"die" laser combo 0 fps

amd ryzen 7
gtx 1050

normally 140 fps+

i have the same problem as necro.
And you get alot of framedrops when minions getting attacked offscreen.

i did a thread about this with a clip.
it seems like the fps drop only, when minions take dmg from mobs, that are on the edge of your screen. And in the last phase you likely run around of the edge and your minions attack and take dmg.

but i don't know if there is a fix for this.
i tried alot of stuff, but so far, nothing helps at all.

Also running into this issue... amd ryzen 3600x, rtx 2070 16 gig ram, the spinning beams drops me to below 5 fps around the time he says die on last phase, instantly dead because the huge fps drops.

Same problem here, deathless until last phase and then cant finish.

OS: Windows 10
GPU: RTX 2070
RAM: 16 GB
CPU: Ryzen 2700x