Computer slowing down when un plugged from the charger.

It's highly possible that the machine is configured for maximum performance when plugged-in and the performance is reduced while on battery to save power.

To change the settings, go to windows settings, system, and change power mode to best performance.  Or, using the legacy Control Panel to create a High Performance power option. 

@Chino de Oro​ I tried all that, I have tried everything. I'm retied now but I was a computer Tech when I was in the work force. I believe it's the battery. I just bought one of Ebay. I'm going to replace the battery, but if you have any other ideas, I'm willing to listen.

Other contributing factor is browser.  Recently, both Chrome and Edge have been employed performance detection settings and changing the system for optimal speed.  But you have described that your apps were also affected, so it was not the case.

Usually, there is a drop in performance if battery power drops below 20% with default settings.  But, if power option was optimized with High Performance, there should not be any change in the timing of opening browser.  Only games and heavy applications would be affected while running with battery. 

@Chino de Oro​ I'm going to replace the battery and let you know what happens.

@RevSarah​ We're having a similar problem with a Precision 5570 - slow as mud when unplugged, heals when powered. We also are retired and were computer support when working (fist bump), so have checked many things already. Battery health reported as "excellent" but I don't believe it. 

Did swapping the battery help this problem? 

thanks. 

Hi, I have the same problem with a precision 5690. The laptop is new , and I tried everything that is in my power to solve this problem but no success. Any ideas? Thanks!

@Jontrisimus Maximus​  It's the Battery I replace my battery, and it fixed the problem.

Another one with the same problem, in my case a 5570 precision. It is not just slow, it is totally unusable, maybe opening windows start menu takes 30 seconds. This is specially bad since December, I would say. 

I am a computer engineer, so I know one or two things. But nothing seems to help, no settings or energy mode seems to make any impact. Only when plugged tothe official charger seems to work. When connected to the monitor through USBC, that is enough to keep the laptop charged, but it seems that now that causes a total degradation 

@RevSarah​, what was your battery life like while unplugged on the old battery prior to replacement? I'm having this issue too, but it just doesn't feel like it should be the battery because I still get ~6 hours unplugged with a full charge.

Dell Latitude 7420

And before anyone else asks, yes, I double checked my power plan settings and everything is set to high performance.

I finally figure out how to fix this by going to  Start > Settings > System > Power & battery  then under the selected plan (High Performance in my case) click on Change Plan Settings > Change Advanced Power Settings which will open a new dialog box. In that box click on Process Power Management > Minimum Processor State and change it for battery to 100% (Mine was set on 5%). 

@RevSarah​ could you please tell me the current situation of your laptop. Is it still working good or problem came back

Could you please elaborate the proper issue like is this laptop very slow that any application such as file explorer also lag

I am asking this because dell service center tell that there is battery will not cause this issue. My laptop is Dell inspiron 3511