I have set blockgeneration time to 15 second but blocks are generated with the speed of 1 block per min(avg) I have forked Litcoin 0.8v to make my own costomized coin(learning purpose), with some changes in parameters:

1. Total no of coins to be mined = 270,000,000. 
2. Block formation time = 15sec. 
3. Time for halving = 2 years. 
4. Life span for mining = 20 years. 
5. Mining reward coins per block for first 2 years = 32.16899445. 
6. Total no of coins to be mined in first 2 years = 135264187.8669276


Every thing is working ok but blockgeneration time seems to be too slow. Though conformation time of 15 sec is just an avg value but in my case i am getting conformation in every 2 to 3 min which is far beyond expected

Edit:I have set my difficulty to be as low as possible which is around 0.002 and retargeting time is set to  10 min, but even after of 3 days of minning i got somewhere around 3400 blocks,and also its difficulty is same in every block though i am doing number of transaction within nodes. 

{
    "version" : 80705,
    "protocolversion" : 70002,
    "walletversion" : 60000,
    "balance" : 17474.98800000,
    "blocks" : 3407,
    "timeoffset" : 0,
    "connections" : 2,
    "proxy" : "",
    "difficulty" : 0.00024414,
    "testnet" : false,
    "keypoololdest" : 1516853983,
    "keypoolsize" : 102,
    "paytxfee" : 0.00000000,
    "mininput" : 0.00001000,
    "errors" : ""
}
Changing just the target block generation time does not mean that the average block interval will be that time. The block interval is governed by the Proof of Work target, and there is a limit to what that target can be. If you do not increase that target's limit, then the target will not be able to increase to the point where your blocks are being mined at the rate you want them to be mined at. You need to increase your target limit (it can be found in chainparams.cpp and is called powLimit) so that blocks can be found more quickly.