Slowness of RT app after upgrade

Hi Team,
I have updated my RT 4 .0.17 to 4.2.15. Now RT is very slow , it is taking 5-10 mins to open one page. Some times it goes on loading.Could anyone please help me on this.

Is it every page or just a couple pages? Maybe enable slow query logging for the database and see if there are any specific queries that are taking a while to run.

I checked, while the pages which fetches something from database, it is taking more time. But in the older version the same query was working fine.

Are the DNS settings correct?

No it is correct only.I checked in the file