It seems if you constantly work with iMac 2019 with i9-9900K, the maximum speed is 3.8ghz due to iMac's poor cooling performance. I dont have an exact data for short uses but this is very concerning result since iMac cannot use high clock speed. Few iMac 2019 from my area reported that iMac 2019 is so hot. Technically, the inner designed didnt change since 2012. I mean the cooling system. What do you expect? i9-9900K requires a high-end CPU cooler and yet iMac has a small fan to cool both CPU and GPU at once which is the worst cooling system I ever saw.
Perhaps I should wait for the next model since Apple didnt update iMac with a new design since 2012? They def need to make a whole new iMac with better cooling system or it wont be able to use all the power from CPU.