Block Time 15 second its a nonsense.
You changed this
DIFFICULTY_TARGET 15 // seconds - before first fork
But not this:
CRYPTONOTE_MINED_MONEY_UNLOCK_WINDOW 60
CRYPTONOTE_DEFAULT_TX_SPENDABLE_AGE 10
MONEY_SUPPLY ((uint64_t)(-1))
EMISSION_SPEED_FACTOR_PER_MINUTE (20)
FINAL_SUBSIDY_PER_MINUTE ((uint64_t)300000000000) // 3 * pow(10, 11)
You know what you are doing with no change the emission or the unlock window/spendable age?, and because 15 second block time its not better than 5 or 10 min?
You only make a bigger (size) blockchain, with a lot orphans blocks, too fast emission (hyperinflation).. and unsecured cripto that any other monero fork.