I have set blockgeneration time to 15 second but blocks are generated with the speed of 1 block per min(avg)

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 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" : ""
}

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