This is most definitely not an issue caused by system memory limitation. We've seen very similar behavior when rendering 6k panorama images in stereo. When the rendering is being displayed on screen, you get a dramatic (2-5x) slowdown in render time. This ONLY happens in stereo renders.

my system specs

nvidia geforce rtx 2070

OS Name Microsoft Windows 10 Home
Version 10.0.18362 Build 18362
Other OS Description Not Available
OS Manufacturer Microsoft Corporation
System Manufacturer Dell Inc.
System Model G5 5090
System Type x64-based PC
System SKU 0947
Processor Intel(R) Core(TM) i7-9700 CPU @ 3.00GHz, 3000 Mhz, 8 Core(s), 8 Logical Processor(s)
BIOS Version/Date Dell Inc. 1.1.2, 2019-11-06
SMBIOS Version 3.1
Embedded Controller Version 255.255
BIOS Mode UEFI
BaseBoard Manufacturer Dell Inc.
BaseBoard Product 0DXJD9
BaseBoard Version A00
Platform Role Desktop
Secure Boot State On
PCR7 Configuration Elevation Required to View
Windows Directory C:\Windows
System Directory C:\Windows\system32
Boot Device \Device\HarddiskVolume3
Locale United States
Hardware Abstraction Layer Version = "10.0.18362.387"
Time Zone Pacific Standard Time
Installed Physical Memory (RAM) 16.0 GB
Total Physical Memory 15.9 GB
Available Physical Memory 11.7 GB
Total Virtual Memory 18.5 GB
Available Virtual Memory 12.2 GB
Page File Space 2.63 GB
Page File C:\pagefile.sys
Hyper-V - VM Monitor Mode Extensions Yes
Hyper-V - Second Level Address Translation Extensions Yes
Hyper-V - Virtualization Enabled in Firmware Yes
Hyper-V - Data Execution Protection Yes

OS and Blender are on C drive, all project files and saves are on D drive

In T73701#868829, @Mike Pan (mpan3) wrote:

This is most definitely not an issue caused by system memory limitation. We've seen very similar behavior when rendering 6k panorama images in stereo. When the rendering is being displayed on screen, you get a dramatic (2-5x) slowdown in render time. This ONLY happens in stereo renders.

It still seems related.
"stereo renders" = two images to render at the same time = more memory required.

@Brian Trepanier (trepaning), without a file it is not be practical to investigate (There are many others reports to investigate and the problem seens to be a limitation in your system memory which is not considered a bug).

@Brecht Van Lommel (brecht), as a cycles developer, do you have any idea what might actually be happening?

In T73701#869216, @Germano Cavalcante (mano-wii) wrote:

In T73701#868829, @Mike Pan (mpan3) wrote:

This is most definitely not an issue caused by system memory limitation. We've seen very similar behavior when rendering 6k panorama images in stereo. When the rendering is being displayed on screen, you get a dramatic (2-5x) slowdown in render time. This ONLY happens in stereo renders.

It still seems related.
"stereo renders" = two images to render at the same time = more memory required.

without displaying the render, it takes 1 minute and 19 seconds to render

Let me add another concrete data point to help with the triaging:

Win10 with 32GB RAM, 2080Ti running Cycles in CUDA mode with Blender 2.81, with renders being outputted to a new image editor window:

Rendering attached file with 1 eye: 5s (so you would expect a stereo render to be ~10s)
Rendering attached file with in stereo with show_stereo_3d on: 45s.
Rendering attached file with in stereo with show_stereo_3d off: 10s

It's clear that displaying the anaglyph view significantly slows down rendering at high resolution.

Thanks for the file Mike, it certainly helps (haven't opened it yet though)

@Mike Pan (mpan3), thanks for the file.
I can confirm that something is wrong with the Show Stereo option.
But I'm still not sure if it is the cause of the 17-minute delay reported (@Brian Trepanier (trepaning), can you confirm).

If it is not the same problem it must be reported separately.

I do not know how to turn show_stereo_3d on and off but

rendering with Blender displaying the render process takes 17 minutes.

Using a BAT** file renders the same image in 1 minute and 19 seconds

BAT content
cd C:\Program Files\Blender Foundation\Blender 2.81
blender -b "D:\TheMusicOfJunk-Rhythm\_FINALS\RENDER2020-360Tets.blend" -a

Mike Pan's test file rendered at 7 seconds as individual,
but 1 minute and 12 seconds as stereo.

Using BAT file, it rendered in 7 seconds.

In T73701#869679, @Brian Trepanier (trepaning) wrote:

I do not know how to turn show_stereo_3d on and off but

Try rendering with and without this option:

In T73701#869685, @Germano Cavalcante (mano-wii) wrote:

In T73701#869679, @Brian Trepanier (trepaning) wrote:

I do not know how to turn show_stereo_3d on and off but

Try rendering with and without this option:

Holy moly, yes sir, that decreased render time to acceptable, thanks.

with display showing render taking place but 3d stereo turned off, this took 1 minute 30 seconds to render.

I had posted this bug before: https://developer.blender.org/T53290 . It seemed to be resolved. ??

That ticket does offer a workaround. By switching to GLSL, I can avoid most of the slowdowns. But it's still slower than expected.

Single Eye: 3s
Stereo Expected: 6s
Stereo GLSL: 8s
Stereo 2D Texture/Automatic: 48s

Interesting that when set to automatic, it seems to pick 2D texture instead of GLSL as T53290 implies.

This seems to be fixed now in (the upcoming 2.91) since (4212b6528afb0) - image draw refactor. Closing it now, let me know if the issue is still around.