Why is my iMac over heating?

iMac has started overheating and causing freezing and slowness.

crwdns2934089:0crwdne2934089:0 crwdns2934093:0crwdne2934093:0

crwdns2934109:0crwdne2934109:0

crwdns2889612:0crwdne2889612:0 0

crwdns2947414:01crwdne2947414:0:

Let's run the onboard diagnostics to check things first. Restart your system and press the D key. Did you get any errors?

Reference: Mac startup key combinations

crwdns2934271:0crwdnd2934271:0crwdne2934271:0

crwdns2934285:0crwdne2934285:0