So recently, I've upgraded the CPU on my Dell OptiPlex 380. It came installed with an Intel Pentium Dual Core E5800. A month later, I decided to upgrade to an Intel Core 2 Duo E8400. Every single site (with the exception of UserBenchmark) I've seen comparing the two, along with a lot of opinions, show that the E8400 is significantly better. How come I'm seeing worse performance? It has a strange bottleneck in less demanding games, and only shows a performance boost in very particular games. Overall, browsing the web is a lot better, I'm able to watch Ultra HD/Quad HD video (sometimes even 4k) and can do general tasks with ease. Despite this however, its gaming performance is extremely lacking compared to my older CPU. How can this be?
Note: I've cleaned my fan and cooler, reapplied thermal paste, cleaned my motherboard, and pretty much the entire system, yet still no performance increase.