This looks like it'll take a little time to read through and get my head around, so I'll probably reply with an update in a week or two

For reference, I'm on a ThinkPad T420S (2011) w/Intel core i7 2640m cpu (2 cores/4 threads - 2.8GHz base clock, 800Mhz min and 3.5GHz turbo).  Jessie amd64 is installed.

I've read a related thread here on d1g about CPU performance and clock speed, and tried using the cpupower tools recommended there - but with little to no effect.

My system is using the intel_pstate governor (via cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_driver) and the problem persists with either performance or powersave profiles enabled (selected through the MATE CPU Freq Scaling Monitor panel widget). 

I can see in System Monitor that multi-thread load ramps up as I mouse over the devtools inspector's elements, which leads me to suspect that as soon as some multi-thread load is present, the clock speed is minimised.  This seems like an overly aggressive power saving strategy to me, as the CPU can't be anywhere near thermal limits. It's also a complete PITA that all the processing power drops away when I need it most!

I have laptop-mode-tools installed and the service enabled, but when I check

service laptop-mode status

I am told

enabled, not active [unchanged]

This is expected as the machine is plugged into AC.

I feel like I'm going mad and have hesitated to post this for the past couple of days, but I could really do with a fresh perspective!  Is there a systematic way to work out why this happens on 'reasonable' multi-threaded load (around 40-50% across all 4 threads), or more importantly, how I can stop this throttling?

Thanks...