strange behavior: it slows down and then recovers

[Cătălin George Feștilă]

my Fedora 30 distro has a strange behavior: it slows down and then recovers to a good
speed...

The hardware :
[mythcat@desk ~]$ lspci 
00:00.0 Host bridge: Intel Corporation Xeon E3-1200 v2/3rd Gen Core processor DRAM
Controller (rev 09)
00:02.0 Display controller: Intel Corporation Xeon E3-1200 v2/3rd Gen Core processor
Graphics Controller (rev 09)
00:16.0 Communication controller: Intel Corporation 6 Series/C200 Series Chipset Family
MEI Controller #1 (rev 04)
00:1a.0 USB controller: Intel Corporation 6 Series/C200 Series Chipset Family USB Enhanced
Host Controller #2 (rev 05)
00:1b.0 Audio device: Intel Corporation 6 Series/C200 Series Chipset Family High
Definition Audio Controller (rev 05)
00:1c.0 PCI bridge: Intel Corporation 6 Series/C200 Series Chipset Family PCI Express Root
Port 1 (rev b5)
00:1c.2 PCI bridge: Intel Corporation 6 Series/C200 Series Chipset Family PCI Express Root
Port 3 (rev b5)
00:1c.3 PCI bridge: Intel Corporation 6 Series/C200 Series Chipset Family PCI Express Root
Port 4 (rev b5)
00:1c.4 PCI bridge: Intel Corporation 6 Series/C200 Series Chipset Family PCI Express Root
Port 5 (rev b5)
00:1d.0 USB controller: Intel Corporation 6 Series/C200 Series Chipset Family USB Enhanced
Host Controller #1 (rev 05)
00:1e.0 PCI bridge: Intel Corporation 82801 PCI Bridge (rev a5)
00:1f.0 ISA bridge: Intel Corporation Z68 Express Chipset LPC Controller (rev 05)
00:1f.2 RAID bus controller: Intel Corporation SATA Controller [RAID mode] (rev 05)
00:1f.3 SMBus: Intel Corporation 6 Series/C200 Series Chipset Family SMBus Controller (rev
05)
02:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express
Gigabit Ethernet Controller (rev 06)
03:00.0 PCI bridge: Integrated Technology Express, Inc. IT8892E PCIe to PCI Bridge (rev
30)
05:00.0 VGA compatible controller: NVIDIA Corporation GT218 [GeForce 210] (rev a2)
05:00.1 Audio device: NVIDIA Corporation High Definition Audio Controller (rev a1)

The RAM memory 
[mythcat@desk ~]$ free -mh
              total        used        free      shared  buff/cache   available
Mem:          1.6Gi       1.1Gi       277Mi       140Mi       280Mi       242Mi
Swap:         3.5Gi       962Mi       2.5Gi

This is a screenshot with htop command running firefox:
https://pasteboard.co/IAGQJ3o.png