Dell Vostro 3470 Running Insanely Slow

First, with an i3 CPU, that is the lowest model Intel CPU.  i5 is my suggestions as a minimum CPU.  

The RAM can be an issue, 4GB is bare minimum and 8GB would be much better.  Dell also ships many PC's (all models) with one memory module and thus no Dual Channel operation.  Adding memory to the second memory slot, along with more memory also gives you the faster Dual Channel operation.

Finally,, you say it worked good initially and then slowed down. That sounds like programs/apps overload.  May be because there is only 4GB of memory.  But too many programs in start up will use up memory and leave very little for operations.  

1) There are no applications other than Adobe Acrobat Reader; MS Office; Microsoft Edge on the PC right now. No heavy application is installed. I even uninstalled McAfee, Dell Support Assist and other pre-installed apps. Agreed that i3 is the bare minimum but with no apps installed and around 900 GB free hard-disk space (out of 1 TB) shouldn't it perform better?

2) I have disabled all apps during startup and cleared all temporary files too. Still it doesn't respond to my mouse clicks instantaneously, lags a lot during web-browsing. This PC was purchased for bare minimum works but it isn't able to perform basic tasks like web browsing. 

Just use SSD in place of hard disk everything will be ok. System will boot approx 12 sec.

re: The PC lags a lot, it does not respond to mouse clicks, Faces difficulties in even single tab web browsing on Microsoft Edge (It is not even able to open Google Chrome  ); not able to run a majority of basic applications like MS Word, PowerPoint.

try run user benchmark test to get a percentile of your pc.  the test takes little time to run.

if it is performing way below expectation based on benchmark, open task manager and look at cpu speed under performance tab.  At idle is the speed way below base frequency of the i3-8100 of 3.6Ghz?

some modern entry level Dell runs processor at insanely slow speed routinely to save power, unless you force the cpu to run at base frequency constantly.   If that is the issue with your pc, you can try download some program to keep cpu run at base frequency.  if that speeds up your pc dramatically, it could be a working solution.

based on your reported pc symptoms, it does not sound like the 4gb memory is the cause.

We have Vostro 3670 model in our company. We have also suffered from the same issue. I can say this problem is not related to the amount of RAM. For Vostro 3670 model, I achieved the issue by modifying BIOS setting. I disabled Intel SpeedStep setting which automatically adjusts CPU frequency for energy efficiency. This setting is located under the Performance menu in BIOS. When this option disabled, CPU performance is at full speed. Restart pc and it is done. (Additionaly you may disable C-State setting which is also located in Performance menu)

That's not the issue.  It does the same thing with additional memory.  It appears that the HDD @7200 RPS is inadequate.  Performance gets worst with each Windows Update.  The sad thing is that the computer is less than 2 years old.  

@timini​ Per your suggestions, I disabled both Speed Step and C-States as well as Deep Sleep and Speed Shift under the Power Management menu. Not sure why anything would be set to save power on a desktop and even these settings for a laptop are questionable as well. Thanks!