What happens to transaction once all the blocks are mined? Once all the bitcoin blocks are mined(21 million limit has been reached) then would calculating the nonce still be so power intensive since the reward at this point of time would only be the transaction fee cost . What are the reasons miners will continue to operate their huge power hogging machines ? Also what would happen to the speed of block creation? Right now it is difficulty which governs it. but once all blocks have been mined how fast would newer blocks be created at the same rate or much faster?
Note that there are not 21 million blocks but 21 million bitcoins generated through mining.  As for your specific questions, the difficulty of a block is independent of its reward so that would continue to match the hashing power of the network.  And the reason that people will continue to mine is, as you state in the question, because they will receive the transaction fees for their effort.