In May 2022 (before Search Layers was added for example) you could have a file with 2600 layers and toggle the visibility on and off in less than a second. Now in June 2023 the same type of file takes 30 seconds to toggle the visibility of a single layer.

Here's how to reproduce the BUG that has been introduced into ILLUSTRATOR now:

I'm using a - Windows 10 Desktop
Illustrator v 27.6.1
ASUS Laptop with 40 GB of RAM
1TB SSD drives and plenty of scratch disk and page file space.

Reproduction Steps:

1. create a new Illustrator document 1920x1080
2. create 100 new blank layers
3. on layer 100 just create a very simple shape like a rectangle
4. copy all 100 layers (so now there are 200 layers)
5. drag the new rectangle (on layer 200) over and give it a different color
6. now copy all 200 layers (so now there are 400 layers)
7. drag the two new rectangles (on layer 300 and layer 400) over and give them a different color
8. now copy all 400 layers (so now there are 800 layers)
9. drag the 4 new rectangles (on layers 500, 600, 700 and 800) over and give them a different color
10. now copy all 800 layers (so now there are 1600 layers)
11. drag the 8 new rectangles (on layers 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600) over and give them a different color
12. at this point you only have 16 shapes (rectangles) each with a different color, but there should be 1600 unique layers in the document (the other layers are just empty and blank for simplicity of this example to to show that it is not the complexity of the art causing the issue)
13. select one of the layers with the visible rectangle in it and toggle the visibility of that simple shape and note how it take many seconds for the visibility of that simple layer to change
14. in MAY 2022 Illustrator build a document far more complex than this simple example had no problem toggling a layer like this in less than a second with over 2600 layers, but now in JUNE 2023 there is a noticeable LAG that starts happening to toggling layer visibility as early as only 400 layers in an Illustrator file

I've attached an example file below so you can see how the final test file should look like in case of issues following the above instructions in order to see and test the bug introduced in the newer version of Illustrator.