If you are basing your firmware on one of the Nordic SDK examples, please note the following lines, typically in main.c toward the top of the file:
#define MIN_CONN_INTERVAL MSEC_TO_UNITS(400, UNIT_1_25_MS) /**< Minimum acceptable connection interval (0.4 seconds). */
#define MAX_CONN_INTERVAL MSEC_TO_UNITS(650, UNIT_1_25_MS) /**< Maximum acceptable connection interval (0.65 second). */
#define SLAVE_LATENCY 0 /**< Slave latency. */
#define CONN_SUP_TIMEOUT MSEC_TO_UNITS(4000, UNIT_10_MS) /**< Connection supervisory timeout (4 seconds). */
These currently default to 400ms minimum / 650ms maximum, and will need to be changed to your desired connection interval (7.5ms min). Note, however, these values are subject to negotiation by the central. iOS tends to allow 20-30ms minimum; Android a little faster.