This has been reported in the posts below for both the 7550 and the 7750. Dell are aware and have advised to wait for a further bios update.

I also had this problem on my 7550 with I7-10850H when my BIOS was upgraded to 1.8.0. Windows Task Manager showed the cpu speed as 0.8 GHz when normally it would show about 4.5 GHz. Really frustrating having to wait all the time, even just to open a folder in Explorer taking about a second when it should be instant. Reverted back to 1.7.0 now and all is running fine, but the Dell Update app is constantly trying to get me to put 1.8.0 back on.

Same issue. Called DELL - got variable support. It appears it is Ok to boot on battery and then plug in the AC only once booted. Have not tried it yet - I rather went in the BIOS (F2 key) and changed a setting in the Performance section. Did not work every time though.

Same issue here! extremely slow. Is there a timeline for an new Bios Update?

Dell Support have told me today:

"Our product engineering team is working on a different BIOS version which should be released by end of this month which will resolve the issue."

I have the same issue.  What I have observed specifically is that the performance with 1.8.0 is fine if the battery is DISCHARGING or CHARGING.  If the battery is fully charged AND the adapter is plugged in, the performance issue appears.  Again, in this situation, I see the bus speed stuck at 0.8 ghz and the cpu utilization never going higher than 31%.  Obviously I bought my 7550 with I7-10850H because I needed the speed (I run a lot of VM's), so if I need a boost I just unplug the power adapter.  Good to know Dell is aware of this.

I had this same problem on my 7750 after installing that "urgent" BIOS update last week. After a week of noticing odd sluggishness, I called DELL Support yesterday and was told there were "no reported problems with BIOS 1.8.0" and that I should try a full driver reinstall using Dell Command Update and then run SmartAssist to optimize my PC. But it was clear to me the person I spoke with had no clue, because when I mentioned that I had saved a restore point prior to the update, she suggested that I could restore that saved point and that would also revert my BIOS back to the prior version (of course it won't because BIOS is stored in flash memory not on the C:). 

So today, I manually downgraded back to BIOS version 1.6.2 (which I had been running previously) and the CPU benchmark is back to where it was before (5x faster than version 1.8.0).

My experience with DELL "ProSupport" has generally been good in the past, but it's frustrating when you get such an amatuerish response instead, and no follow-up whatsoever.

hI Same issue with my 7550 i7  in combination with a ucs c hub connected, after sleep of lock the cpu goes down to 0.8x and lt gets slow and does not recover . but if i remove the power cord en put it back i have full cpu and speed again.  irritating but workable 

It seems that the 1.8 update is now blocked for download.

Slightly annoying that it is still being recommended by SupportAssist and other update suggesting options. Also that I have wasted some time trying to download since there was no contrary advice despite running repeated checks.

And then 2 attempts to create a Support Request (before realizing I could check here and have some success ...) both seemed to hang totally when the Submit button was applied. 

At least today is a Friday ... so it's not all bad.