If the computer still is sluggish:

Run the diagnostic tool to check Hardware on the system by following the steps below:

1. Power off system. Remove all peripherals (except AC adapter).

2. Power on system, quickly start tapping F12 key continuously until you see Preparing one time boot menu in the top right corner of the screen. In boot menu, stroll down to Diagnostic and click on it. After quick tests, click on hard drive, and then Thorough Test Mode. After hard drive test finished, select Memory and Thorough Test Mode.Report back any error code.

ePSA Pre-boot System Assessment (Official Dell Tech Support)

Please power off system. Remove all peripherals, and AC adapter. If the system is receiving WIFI / Internet from a cable modem or router,unplug the power cables for the router / cable modem from the wall outlet and then wait a few minutes. Plug the power cables back into the wall outlet and replace AC adapter. Please wait a few minutes for the modem and router to fully power on. After, power on system, hopefully computer is a lot faster.

I have no access to the modem and router as they are in the unit next door and the landlord isn't there. That said, it's hard to see how that could be the problem anyway. It's the computer that's slow, not the network. Web pages open quickly. Some programs open slowly and they run slowly. For example, with nothing else open or running, File Explorer takes 5 full seconds to open. It should be instantaneous. Very strange.

I ran the full suite of diagnostics as requested. There were no errors found. All tests passed. 

1) Recent updates, that "fixes"  issues with Meltdown and Spectre attacks for Intel processors may cause some performance issues. There are apps that allows to check that those updates were applied on your machine and allow to disable/enable them to verify whether they caused any problems (ie. program named GRC Inspectre).

2) Check (take a look at process manager and system stats) whether some app running in the background is not  heavily eating your CPU/RAM?

3) Scan your PC, to check for virues, or some cryptocurrency miner running in the background. Try to use other antivirus software than right now (if you use any).

4) Its good practice to perform clean Windows installation from time-to-time; After 1 year+ it typically runs slower than at the beginning.

You using Chrome as your browser? You tried a different browser to see if the issues still occurred? 

If you have the Killer Wireless WLAN card installed change it or get Dell to change it. I have had the same sluggish issues: Chrome slow to open pages; browsing in general was slow; slow programmes loading; slow booting.

This morning I replaced the Killer Wireless WLAN card with an Intel 8265 card and now the whole system is significantly quicker in every way: booting; programmes loading; browsing the internet (I use Chrome but tested FF and Edge - all worked far more quickly).

• Download Intel Extreme Tuning Utility from here

Intel Extreme Tuning Utility

• Click on 'All Controls' tab on the left.

• Set the 'Turbo Boost Power Max' slider to 45 (the TPD of your processor)

• Click on yellow 'apply' on right.

• Watch in amazement as your laptop stops performing like a tablet.

I just purchased a Dell Inspiron 7577 and the cpu was set to 5.5 instead of 45, which was the cause of the lag and terrible performance.