XPS13 (9343), intel_pstate: stuck to minimum cpu-frequency

Hi,

analyzing a performance issue (not knowing whether my expectations are too high
but assuming performance is too low...) I had to realize that cpu-frequency seems
to be stuck to it's minimum (500 MHz) all the time.

1) Environment:
    Dell XPS13 (9343)
    Intel i5-5200 CPU@2.20 GHz
    BIOS A05
    Intel Wireless 7265
    intel_pstate

    Ubuntu 15.04  / 14.04 LTS
    Kernel: several versions,
                most of the time 3.19.0-28
                but also tested with 4.0.0., 4.2.0

2) How did I check the scaling_driver?
    cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver
    intel_pstate

3) How did I check the cpu-frequency?
    a) lscpu |grep MHz
    b) sudo watch -n 0,3 cat /sys/devices/system/cpu/cpu{0..3}/cpufreq/cpuinfo_cur_freq
    c) cpufreq-info
    d) powertop (Tab to "Frequency stats")

    On XPS13 powertop displays only two frequencies in "Frequency stats":
    500 MHz, 600 MHz.
    On a different notebook (booted with the same  USB-stick) "Frequency stats"
    shows a whole bunch of different frequencies, from min-frequency up to max-frequency
    (min-/max-freq. as reported by lscpu).

4) How did I try to influence the (stuck) cpu-frequency?
    a) by creating workload
    b) by increasing minimum cpu-frequency:
       cat /sys/devices/system/cpu/intel_pstate/min_perf_pct
       18
       echo "30" | sudo tee /sys/devices/system/cpu/intel_pstate/min_perf_pct

       On the other notebook, booted with the same USB-stick, this changes
       the cpu-frequency immediately to a higher value.
       You can see cpu-frequency scaling up/down; it looks like not undergoing the
       frequency defined via min_perf_pct.

       On my XPS13 the CPU-frequency is tumbling around 500MHz,
       mostly 499,984 MHz never higher than 500,240 MHz.
       Trying to change CPU-frequency as described fails - no reaction at all.
       The frequency always remains at ~500MHz.

5) I have tested with several kernels, same results/behavior

     Exception: With kernel 4.2.0 powertop two counted times showed additional
     frequencies to 500 and 600 MHz: 1.100MHz, 2,7 GHz.
     I can not reproduce this , it  did not happen by creating workload like starting
     a program, etc. During program starts (Firefox, Writer, VMware-player...) the
     frequencies displayed are  only 500, 600 MHz.
     The system was pretty idle, Firefox was running, displaying the firefox-standard
     -startpage (google) when additional frequencies have been displayed for a moment.

6) Strange or confusing:
    (Only) when running the system with acpi-cpufreq driver CPU frequencies are not stuck.
    Under load they go up to  ~2.2 GHz. However, the performance does not increase
    as one would expect.
    (Pl. see:
en.community.dell.com/.../20824758
    and below)

Questions:

a) Do I have a problem of understanding?
b) Could the stuck-to-min-cpu-frequency be a hardware issue? How likely is this?

Thanks for any hint / support!