“Slow” system & ORA-00445: background process “DIAG” did not start after 120 seconds ?

Since I am currently checking all Oracle systems at the customers site, I thought at first sight, that almost all of them are overutilized or wrong configured, because everything seemed so sloooooow… Especially the new logins and creating of the new processes were a way too slow – more than 5 seconds! Sometimes I waited ...