Here is how several cpus compare in Puget Systems’ After Effects benchmark when tested with identical components (RAM, SSD, GPU, PSU, etc) and at stock settings (no overclocking, no unlocked power limits).
As you can see, past 8 or 10 cores, After Effects does not scale with core count. Past 8 or 10 cores, the cpus differentiate from one another based on their per-core performance dictated by IPC (which in some cases can be benefitted by additional cache) and clock speeds when 8 cores are active. Also, under stock conditions, the 11900K scores 1426 and the 10900K scores 1341. The score for the 12900K being 1575 is therefore 10.4% higher than that of stock 11900K and 17.4% higher than that of stock 10900K. These are decent gains over the last flagships for After Effects. It is also 4.1% faster than the 5900X and 5950X.
Also bear in mind here that this is a qualification sample, and we don’t know at what clock speed it was running at (it might have been running at lower than final stock frequencies even if it was overclocked relative to its current stock frequencies). Also revised microcode and revised stepping can easily increase performance further. We might see a score higher than 1600 with final retail units at stock settings and higher than 1700 with adaptive boost.