It can happen that your system is running fine, however a service, set of services it is calling while processing an event like button click, tab switch etc. may be taking time.
We faced the same kind of scenario, in which initially it seemed that the application is running slow because of multiple threads firing up at same time or too much logging. However, on doing more analysis we found that when a process engine task was sending an email synchronously then it was taking a lot of time, sometimes even failing to send the mail. It happened that other teams were also observing slowness in sending mails. This was occurring due to some issue at the SMTP server end. With the issue being resolved at the SMTP server end the slowness was gone.
We further optimized by making the email sending process asynchronous.