Mysql suddenly become slow for different connections to changes

[Wayne John]

Hi,
I have a web application. It is a java-jsp-tomcat-mysql. It is working, but since yesterday, it became slow. For each data update, it is not slow: successful page shows promptly. But the next page (it uses a different database connection from tomcat.DataSource connection) does not see updated data. I wait a coupe seconds, refresh the page, the updated date shows. This issue never happen before. I checked server cpu and memory. It is okay.

The server runs on Ubuntu: 4.15.0-54-generic, with mysql: 8.0.21.

What can be wrong? Any information would be appreciated. Thanks in advance.