vmware – What is reason (CPU, memory or hard disk) that cause my Ubuntu machine slow?

I am running docker on a virtual Ubuntu 18 server (provided by a VMware Virtual Platform). Sometimes when I run docker stats, it updates very slow (the result show dashes instead of containers info, example in following images slow0 slow1). When I type a command in a terminal, it is slow too.

I tried htop, the slow server has a lot of CPU and memory remaining (example htop0).
I tried iotop, the slow server’s disk IO is not greater than another normal server.
What can I do to determine why the server is slow?