There could be any number of reasons for poor gaming performance. Typically if the issue is only noticed in gaming then it's a GPU driver issue. If you're not using the latest drivers you should update and see if things improve. If the issue is a system wide issue then you may have other issues beyond drivers. How well gaming software is written also has a major impact on gaming performance. Most gaming software is only written for 4-core CPUs, meaning more than four cores results in under utilization of the extra cores. Software that is properly written to use all cores results in a noticeable improvement in system performance.
Main glaring reason for bad performance in games with the specs you've listed would be the HD 5450. It's a very low end GPU mostly useful for HTPCs rather than gaming.
According to techpowerup (ATI Radeon HD 5450 PCIe x1 | techPowerUp GPU Database ) it doesn't have enough ROPs or Compute Units to run just about any games made in the last few years, you could probably only really get good performance from games made in the Voodoo card era, even games released in 2010 when that card was new were beyond it's abilities, from what I can gather looking around you're lucky if you can get 30fps at 1280x720 with low to medium settings on many contemporary games
Yep, old low end graphics card is the reason.