Q:

How can we reduce the transaction time in ethereum?

I created a single node using geth --dev and tried transacting between two personal accounts using: eth.sendTransaction({from: 'first_acc', to: 'second_acc', value:web3.toWei(1, "ether")})
This however takes a long time. How can I reduce the time required for the transaction? 

A:

Currently the average blocktime is ~ 14 s where a significant portion is just ~ 1 s. See my empirical asssesment here: https://github.com/rolandkofler/blocktime
 
If you run your private net you can change the blocktime or difficulty algorithm, but if you have only two nodes it should be rather fast anyway so I wonder if your setup has problems.
If you run on the public net then probably the best is to spend more transaction fees so that your transactions will be mined with higher priority. 
With Casper, the Proof-of-Stake Algorithm, block times are expected to halve again and be around ~ 7 s

