Is there any way of stress testing a dapp? I developed dapp, now I want to see how much stress it can handle? I can't test it on Main Ethereum Network. At least I can do it on Rinkeby test Network. Also, will there be any difference in processing time in the case of Main ethereum network and Rinkeby test Network
There is no point in stress testing a dapp. The speed depends mostly on factors such as network congestion, given gas price and transaction size. In any case the speed can't be compared between measurements - it may vary wildly and everything is still ok.
Furthermore, different test networks are not the same as the main network. Even if you managed to complete some sort of stress tests in testnets, the results would be probably quite different in the mainnet. The parameters - and especially usage - of the networks are different.
The biggest delays in smart contract execution are not caused by code quality but by the network itself. So optimizing the code makes no difference for speed, but of course it may make a big difference for execution cost (gas).
In the end, assuming the contract(s) do what they intend to do and are secure, gas is the only measurement you should be interested in.