I loaded a recent project which performed perfectly in VP17 into VP18. Project was 32-bit Floating Point and had a mix of Canon XA10 footage, stills and Mavic Pro 4K drone footage. GPU was initially ON. Playback was poor, even in Preview=FULL mode in VP18, sitting around 20-23fps. When it encountered the 4K footage playback fell to around 5-7fps. Then it crashed, turning white then needing to be closed.
I did several tests with GPU=ON and GPU=OFF but it still crashed, but not always on the drone footage. Sometimes on the Canon footage.
Switching to 8-bit was better but GPU=ON still crashed every time. I could get 25fps at BEST, FULL preview on the XA10 footage and the 4K drone footage did play back at Preview FULL, but even at these lower settings VP18 still crashed, even with GPU=OFF.
Win 10 64-bit
Ryzen 7 2700X @3.7GHz
nVidia GeForce GTX 1660 Super with 6GB DDR6 RAM
As always, I NEVER start using the latest version of Vegas on 'real' projects until bugs like this have been solved.
This is a real shame because there are so many excellent new features in VP17 and 18.