Intel Core i7 9800X 's Advantages
Larger memory bandwidth (79.47GB/s vs 76.8GB/s)
Higher base frequency (3.8GHz vs 2.4GHz)
Intel Core i9 12900 's Advantages
Released 3 years and 3 months late
Higher specification of memory (DDR5-4800 vs DDR4-2666)
Newer PCIe version (5 vs 3.0)
Larger L3 cache size (30MB vs 17MB)
More modern manufacturing process (10nm vs 14nm)
Lower TDP (65W vs 165W)