Recently I have seen multiple instances (fairly rarely, but nevertheless) of the newer Dell Latitude Ultrabooks (circa 2013/2014 models, E7440 and E7240 specifically) throttling CPU frequencies under exceptional power conditions (such as possibly a misbehaving AC adapter or extremely low battery condition while under load). I haven’t confirmed the exact circumstances which lead to this behavior, but I do know of a solution.
I first noticed this when a client recently reported sluggish operation of his brand-new E7440 Ultrabook… which, of course, made little sense considering the blazingly-fast parts (SSD included) that we purchased for him. I checked the software briefly and saw no issues which would suggest configuration problems. However, upon opening Task Manager, under the Performance tab, the CPU frequencies were reportedly below 400 MHz permanently–which, of course, is incredibly low considering the max Turbo Boost frequency of the i5 Haswell CPU he had of 2.8 GHz. Fortunately, I had seen this problem once before.
My theory is that it is likely related to power disruption conditions, as I have only thus far seen it happen in circumstances where an AC adapter was not providing proper voltage or where the machine was in a very low battery state while sustaining heavy CPU loads for some reason (Windows Updates, etc.). The machine responds by throttling CPU clock rates to protect itself from possible damage, but the problem is that it never reverts from this throttled state until it is powered off and the battery is removed.
Fortunately, the solution is easy, if not a bit difficult to discover. All that is required is a BIOS update to the latest firmware available from Dell (support.dell.com, search for your particular model). In my most recent client’s case, an upgrade from A05 to A15 immediately corrected the problem. It remains to be seen whether it recurs, but I do not expect it to given the last instance I saw, where we did just the same thing and the problem was permanently corrected.