I've had this exact same problem with my Dell Precision 5510 and Xeon 1505-M processor. Despite many different startup & BIOS adjustments, I haven't been able to correct the issue. Did you find a solution to this problem?
A quick followup from by previous post. I was able to solve this issue on my computer with the following steps:
01. Reset BIOS settings to factory settings, runs CPU=0.8Ghz. (no change, in other words, but mentioning the step here just in case there's a connection)
02. Opened the case and noticed the internal battery to MB connector cord was just slightly loose on one side.
02a. Disconnected battery and ran on AC only, CPU=2.8Ghz+ (throttled slightly above 2.8 with turbo boost settings)
02b. Reconnected battery and tested on AC and battery both, CPU=2.8Ghz+
02c. Seems a loose battery cable was the culprit.
I hope you've been able to resolve your issue by now sinsidet. One weekend of very slow performance was more than enough for me.
How were you able to pull down older versions of the bios? My computer ran a bios update and now my cpu is being throttled at 0.8. I reset the bios settings back to defaults and checked all connections inside. Unfortunately, everything seems fine.
I did not attempt to install an older version of the BIOS, only reset to factory settings. You may have already done this, but verify the individual settings within BIOS (processor speed, # of cores) to ensure that factory settings are what you expect them to be. From your description it seems like you may have a different culprit and I do not have any additional suggestions.