With 24GB of RAM, you will operate on dual channel on only 16GB of the total. (the 8GB module and 8GB of the 16GB module). Plus there is a good chance that the modules are not fully compatible based on the make of each module and the module speed and type. The best way to upgrade RAM has always been to install a matched kit of identical RAM modules. Also you did not specify the speed of the RAM. If they are not the same speed then the RAM will run at the speed of the slowest module.
  Hello, I've recently upgraded my RAM on a DELL 7400 from: - 8 GB TOTAL RAM (1x 8 GB, 1 slot in use) - 24 GB TOTAL RAM (1x 8 GB + 1x 16GB, 2 slots in use) This resulted in a noticeable decrease in performance, when compared with the initial setup (only 8 GB). This is of course not expected. Removing the 8 GB stick, while keeping the newly introduced one i.e., having: - 16 GB TOTAL RAM (1x 16GB, 1 slot in use) resulted on a significant performance improvement, when compared with the initial setup (only 8 GB), which is expected.   So, are there any known issues related to the described RAM upgrade (from 8GB to 24 GB RAM)? Is it a requirement to have both slots with the same RAM capacity? Grateful for any advice. Kind Regards.
Dell 7400 RAM upgrade