but the problem here is that the file plays smoothly, only when I turn on the color grading page (without adding any effects) do the files start to lags. 
ps. The project is set to 8 bit.

I can't say I am experiencing that issue here on my computer (see Sig). The footage I used was from Panasonic camera, UHD4K 10 bit 4:2:2.

Playing by DJI Mini 3 Pro D-Cinelike 4K footage, after doing some grading, again, switching back and forth with the Color Panel, no delays. Timeline playback would go from 25fps to 17fps with color grading enabled. Preview setting is Best>Full.

More information is needed. Specifically your computer specs. How about adding them to your User Profile? Second, what created the footage and can you provide a sample?.

Also, it is always best to read this POST and include the necessary information from "B" and "C" in your post.

All right. 
But the problem is that the preview works very well at 25fps. 
When color grading panel is started without any correction, the image starts to freeze / lags. 

Vegas 19, with the same settings, same corrections, with color grading panel, works very well, no problems. 

But in Vegas 21, just running the color grading panel causes lag without adding any correction, this is clearly visible in the attached video in 1st post.

So it's not a hardware issue.
If I remember correctly, I didn't have this problem in V21 build 208.

translated

When color grading panel is started without any correction, the image starts to freeze / lags. 

Vegas 19, with the same settings, same corrections, with color grading panel, works very well, no problems. 

But in Vegas 21, just running the color grading panel causes lag without adding any correction, this is clearly visible in the attached video in 1st post.

So it's not a hardware issue.
If I remember correctly, I didn't have this problem in V21 build 208.

translated

 

I was using VP22 for the screencap, I tried VP21b315 and it does have worse playback at Best/Full for a 4K video in 8bit mode.

Still have zero idea of what type of computer hardware is being used or what is the source video... @VegasUser I pointed you to a post that is designed to help others help others. Without that information.....

V21
Windows 10
i7-7700HQ 2,8 GHz, 8-core
NVIDIA GeForce GTX1060 6 GB
32 GB RAM

V21
Windows 11 PRO
Snapdragon X Elite 3,4 GHz, 12-core
32 GB RAM

- DJI Mavic 2 / 4K 25fps HEVC
- A7S III 4:2:0 10bit 4K50p HEVC
- A7 IV 4:2:0 10bit 4K50p HEVC
 

However, I will add that the problem occurs only and exclusively when the "Color Grading Panel (Alt+G)" is enabled.
Without adding any correction, the image still stutters and lags.
When I disable the Color Grading Panel, everything works fine.
I’m not even mentioning what happens when I apply a correction—then playback drops to 1-2 fps, and rendering a one-hour clip in 4K 40Mbps video takes over 16 hours :)

P.S. The same settings, the same correction in V19, everything works perfectly—rendering takes about 2 hours, and playback runs at around 16 fps in 4K Best (Full).

I will also add that even when I create proxy files, which I use, the situation is exactly the same. Enabling the Color Grading Panel in V21 causes the image to lag.

The Vegas Color Grading Panel is very slow, as I mention in VP22 in 32bit resolution it can't playback 4K smoothly, need to playback in Best/half, it can however playback smoothly for 4K24p in 8bit. Maybe your GPU is a problem but not sure, the CGP needs more GPU power than non CGP. But you mention everything worked fine in b208 which still hints at b315 is the problem