Describe here the kind of symptoms, then process to the according diagnosis.
Check memory usage (sorted)
ps -o pid,user,%mem,command ax | sort -b -k3 -r
Clear RAM Memory Cache / Buffer / Swap space:
free -m && sync && echo 3 > /proc/sys/vm/drop_caches && free -m
Linux IOwait: https://madflojo.medium.com/troubleshooting-high-i-o-wait-in-linux-358080d57b69
Global view
top & htop & glances
Which disk is writing/reading/...
iostat 2 50
Find which process causing I/O
apt install -y iotop;iotop
On my side I got the following processes that were causing IO>
Arpwatch: https://www.tecmint.com/monitor-ethernet-activity-in-linux/