This is somewhat long. I wanted to provide all the info. I have been working on this for a while...
Product Name: Inspiron 5558 System BIOS: A17 Service Tag: ePSA: Build 4302.10 UEFI ROM OS Name: Microsoft Windows 10 Home Version: 10.0.17134 Build 17134
1) The behavior started many weeks ago on a previous install of Windows 10. The computer was very slow, to a point that it was barely usable: typing and waiting for letters to appears. I thought the computer was slow because of software or operating system issues. I proceeded with a fresh re-install of Windows 10. The slow behavior was still present after the re-install. On this fresh install, only an anti virus (Avast) and Firefox is installed.
2) By reading on the issue on the internet, I realized that the CPU speed was decreased to 0.48Ghz when it should be 2.2Ghz (Inspiron 5558, Core i5-5200U). I also realized that the battery icon on task bar tray showed 'Plugged In, Not Charging'. My battery level is 42% and is not decreasing or increasing when the laptop is plugged in. The front light (which should be white when charging) is not lit.