ubuntu 18.04 high cpu/fan crazy - basic usage

I've done a fresh install of ubuntu 18.04. My system is the following:

lucas@test:~$ inxi -F
System:    Host: test Kernel: 4.15.0-20-generic x86_64 bits: 64 Desktop: Gnome 3.28.1 Distro: Ubuntu 18.04 LTS
Machine:   Device: laptop System: Hewlett-Packard product: HP ENVY 15 Notebook Pc v: 09A2100000400000000000000 serial: N/A
           Mobo: Hewlett-Packard model: 228D v: 77.12 serial: N/A UEFI: Insyde v: F.04 date: 05/15/2014
Battery    BAT0: charge: 43.2 Wh 100.0% condition: 43.2/43.2 Wh (100%)
CPU:       Dual core Intel Core i7-4510U (-MT-MCP-) cache: 4096 KB
           clock speeds: max: 3100 MHz 1: 1327 MHz 2: 1084 MHz 3: 1000 MHz 4: 982 MHz
Graphics:  Card-1: Intel Haswell-ULT Integrated Graphics Controller
           Card-2: NVIDIA GM108M [GeForce 840M]
           Display Server: x11 (X.Org 1.19.6 ) drivers: modesetting,nvidia (unloaded: fbdev,vesa,nouveau)
           Resolution: [email protected], [email protected]
           OpenGL: renderer: GeForce 840M/PCIe/SSE2 version: 4.6.0 NVIDIA 390.48
Audio:     Card-1 Intel 8 Series HD Audio Controller driver: snd_hda_intel Sound: ALSA v: k4.15.0-20-generic
           Card-2 Intel Haswell-ULT HD Audio Controller driver: snd_hda_intel
Network:   Card-1: Broadcom Limited BCM43142 802.11b/g/n driver: wl
           IF: wlo1 state: up mac: d8:5d:e2:cc:38:5d
           Card-2: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller driver: r8169
           IF: eno1 state: down mac: 70:5a:0f:1c:fb:cd
Drives:    HDD Total Size: 1000.2GB (45.1% used)
           ID-1: /dev/sda model: ST1000LM024_HN size: 1000.2GB
Partition: ID-1: / size: 228G used: 6.0G (3%) fs: ext4 dev: /dev/sda2
           ID-2: /home size: 681G used: 407G (64%) fs: ext4 dev: /dev/sda3
           ID-3: swap-1 size: 8.00GB used: 0.00GB (0%) fs: swap dev: /dev/sda4
RAID:      No RAID devices: /proc/mdstat, md_mod kernel module present
Sensors:   System Temperatures: cpu: 65.0C mobo: N/A gpu: 38C
           Fan Speeds (in rpm): cpu: N/A
Info:      Processes: 276 Uptime: 41 min Memory: 3103.8/15970.9MB Client: Shell (bash) inxi: 2.3.56 

Nevertheless, I get high CPU spikes from time to time eventough I really do use it so far for web browsing and libreoffice. With htop I get to see that:

• Firefox consumes most of the CPU which makes the fans of the laptop go crazy.
• doing apt-get upgrade makes CPU go high
• playing youtube videos above 50% of CPU
• etc ...

Is there anything I can look at to find out what's going on?