So yeah, I recently purchased a Lenovo IdeaPad 5 Pro which has AMD Ryzen 5 5600H processor with ADM Graphics. Running Windows 11. I used Vegas Movie Studio 17 Platinum with my previous PC with almost no problems at all. With my new computer, I tried to render my video with the settings I've always used: Sony AVC/MVC Internet 1920x1080-30p. All my videos that I've been editing share the same 30p frame rate, so I think that is not the problem. I tried other rendering settings too, still the outcome is the same. The preview works fine with the settings set to 'best'. So I guess it's something with rendering. I've tried every step on NickHope's Vegas Pro troubleshooting 'Fixing Quality Problems' (https://www.vegascreativesoftware.info/us/forum/faq-why-does-my-rendered-video-look-bad-troubleshooting-quality--103361/) but nothing won't help.
I seriously can't think of any other problem than the processor. On my previous computer, the processor was Intel core i7, which was automatically selected as 'GPU acceleration of video processing' from the preferences. The video would always play smoothly after rendering. I tried to turn off the GPU acceleration too on my new PC but it won't help.
If I try to explain the 'lagging' more specifically, the video looks like it's been slowed down a little, and all the quick movements tend to lag.