CPU systematically scaled down

Following an upgrade (or a reboot) on a Debian testing, I noticed that my system was much slower than usual. The CPUs seem to be systematically scaled down. I used cpupower to ensure that the performance governor was used:

analyzing CPU 1:
  driver: acpi-cpufreq
  CPUs which run at the same hardware frequency: 1
  CPUs which need to have their frequency coordinated by software: 1
  maximum transition latency: 10.0 us
  hardware limits: 400 MHz - 2.70 GHz
  available frequency steps:  2.70 GHz, 2.70 GHz, 2.60 GHz, 2.50 GHz, 2.40 GHz, 2.30 GHz, 2.20 GHz, 2.10 GHz, 2.00 GHz, 1.90 GHz, 1.80 GHz, 1.70 GHz, 1.60 GHz, 1.50 GHz, 1.40 GHz, 1.30 GHz, 1.20 GHz, 1.10 GHz, 1000 MHz, 900 MHz, 800 MHz, 700 MHz, 600 MHz, 500 MHz, 400 MHz
  available cpufreq governors: userspace performance schedutil
  current policy: frequency should be within 2.70 GHz and 2.70 GHz.
                  The governor "performance" may decide which speed to use
                  within this range.
  current CPU frequency: 2.70 GHz (asserted by call to hardware)
  boost state support:
    Supported: yes
    Active: yes

However despite what is claimed, the CPU is not running at 2.7 GHz. Here is the result of turbostat --Summary --quiet --show Busy%,Bzy_MHz,Avg_MHz --interval 1 with a running Firefox and VLC:

Avg_MHz Busy%   Bzy_MHz
92      18.59   494
59      12.64   469
52      10.11   512
164     37.58   438
184     43.01   429
261     56.64   458
225     44.50   505
244     45.91   534

The CPUs are quite busy but running at a slow frequency. I tried to stress the CPU with a CPU-intensive program: I launched xz -T 10 on a huge file. Here is the output of turbostat when xz was running:

493     98.71   499
497     99.77   498
496     99.61   498
497     99.35   500
495     99.38   498
489     99.76   490
489     99.70   493

We see that the CPU is fully used but still running at a low frequency. In the meantime, using sensors I can see that the temperature of the CPU looks normal:

coretemp-isa-0000
Adapter: ISA adapter
Package id 0:  +53.0°C  (high = +100.0°C, crit = +100.0°C)
Core 0:        +46.0°C  (high = +100.0°C, crit = +100.0°C)
Core 4:        +47.0°C  (high = +100.0°C, crit = +100.0°C)
Core 8:        +51.0°C  (high = +100.0°C, crit = +100.0°C)
Core 9:        +51.0°C  (high = +100.0°C, crit = +100.0°C)
Core 10:       +51.0°C  (high = +100.0°C, crit = +100.0°C)
Core 11:       +51.0°C  (high = +100.0°C, crit = +100.0°C)
Core 12:       +49.0°C  (high = +100.0°C, crit = +100.0°C)
Core 13:       +49.0°C  (high = +100.0°C, crit = +100.0°C)
Core 14:       +50.0°C  (high = +100.0°C, crit = +100.0°C)
Core 15:       +50.0°C  (high = +100.0°C, crit = +100.0°C)

Thus the CPU throttling doesn't seem to be caused by a high temperature. What could be causing this and how could I change the setting? Thanks a lot!