Yes, there program really sucks now, I also upgraded from Premium 2020 to 2021 and when I drag a video to a track it lag, it's suppose that with the new INFUSION Engine 2 all the rendering is made in the video card, just BS, I got a nvidia GeForce GTX 1650 with 4gb, a AMD Ryzen 3 and 16gb memory system with windows 10 and the only thing lagging in my computer is Magix Video Premium 2021, a really waste of time and money, lesson learned never upgrade so soon.
Yes. the behaviour of MEP 2021 seems a lot different to the way 2020 behaved.
I had the same problems at first but now I'm finding it faster on my machine but I had to set up the program and Windows differently than before to get the desired performance difference. I don't think that is guaranteed for all systems but altering settings and then looking at Windows task manager will show up which parts of a computer is being used help to work out to what effect on both playback and export. I think this time around that different systems will need different settings to get the best performance from MEP that they can. The program seems much more dynamic in that sense than it was.
It may not all be down to MEP changes though. Windows 10 (remember, the last version of Windows ever as promised?) now dynamically alters as updates come out. It has even removed or stopped programs from running as computer component drivers become outdated or no longer supported by alterations made within Windows. So while individual components may still work within a system their functionality within the system will diminish over time.