Not sure if this is because of the upconverted AEP project file from CS5 or because of After Effects CC actually rendering slower, but I had a project started in CS5 take over 2x longer to render out in CC 2021 at 5 hours instead of 1.5 hours. I thought maybe an update to CC 2022 would help, but then the same project file ended up taking 13 hours instead.

Switched computers just in case and the results were the same. The project is pretty simple - just a PSD with layers broken out and slightly randomized movement for that animated look. The project opens and edits in After Effects just fine - it's only the rendering that takes forever in the newer versions. It appears to be related to how PSD layers are imported, since deleting the PSD layers removes the difference in rendering time; the PSD has only basic layers and no smart objects.

Export is through Adobe Media Encoder with the below settings:
Resolution 1920x1080
Format: H.264 MP4
Bitrate: VBR 1-Pass 12Mbps
Length: 2 min, 50 sec

PC specs are:
Windows 10 Pro 64-bit
i7-9750H 2.6Ghz CPU
32GB RAM
RTX 2060 GPU

Attached is a stripped-down version of the project with empty layers, so it's easier to test, but it exhibits the same problem. I have all the PSD layers cleared so there's no actual image, but it still has the performance hit. On CS5, it renders reasonably in 10 minutes. On 2021 it takes 30 minutes, and on 2022, it takes at least an hour - not sure on the final count since it's still rendering and less than 50% done.

Download test project here:
https://drive.google.com/file/d/1NW8cPQRbl3yMt9qCU0gLOodqYA_ZW3AR/view?usp=sharing