SAMC21 FAT slow on samc21
@kartman was right. The clock was not configured correctly.
This is my new conf_clocks.h
#include &lt;clock.h&gt;
#ifndef CONF_CLOCKS_H_INCLUDED
#  define CONF_CLOCKS_H_INCLUDED

/* System clock bus configuration */
#  define CONF_CLOCK_FLASH_WAIT_STATES            2
#  define CONF_CLOCK_CPU_DIVIDER                  SYSTEM_MAIN_CLOCK_DIV_1
    
/* SYSTEM_CLOCK_SOURCE_OSC48M configuration - Internal 48MHz oscillator */

#  define CONF_CLOCK_OSC48M_FREQ_DIV              SYSTEM_OSC48M_DIV_1

#  define CONF_CLOCK_OSC48M_ON_DEMAND             true

#  define CONF_CLOCK_OSC48M_RUN_IN_STANDBY        false



/* SYSTEM_CLOCK_SOURCE_XOSC configuration - External clock/oscillator */

#  define CONF_CLOCK_XOSC_ENABLE                  false

#  define CONF_CLOCK_XOSC_EXTERNAL_CRYSTAL        SYSTEM_CLOCK_EXTERNAL_CRYSTAL

#  define CONF_CLOCK_XOSC_EXTERNAL_FREQUENCY      12000000UL

#  define CONF_CLOCK_XOSC_STARTUP_TIME            SYSTEM_XOSC_STARTUP_32768

#  define CONF_CLOCK_XOSC_AUTO_GAIN_CONTROL       true

#  define CONF_CLOCK_XOSC_ON_DEMAND               true

#  define CONF_CLOCK_XOSC_RUN_IN_STANDBY          false

#  define CONF_CLOCK_XOSC_FAILURE_DETECTOR_PRE    SYSTEM_CLOCK_XOSC_FAILURE_DETECTOR_PRESCALER_1

#  define CONF_CLOCK_XOSC_FAILURE_DETECTOR_EVENT_OUTPUT_ENABLE  false

#  define CONF_CLOCK_XOSC_FAILURE_DETECTOR_ENABLE  false

#  define CONF_CLOCK_XOSC_FAILURE_SWITCH_BACK_ENABLE  false



/* SYSTEM_CLOCK_SOURCE_XOSC32K configuration - External 32KHz crystal/clock 
oscillator */

#  define CONF_CLOCK_XOSC32K_ENABLE               false

#  define CONF_CLOCK_XOSC32K_EXTERNAL_CRYSTAL     SYSTEM_CLOCK_EXTERNAL_CRYSTAL

#  define CONF_CLOCK_XOSC32K_STARTUP_TIME         SYSTEM_XOSC32K_STARTUP_65536

#  define CONF_CLOCK_XOSC32K_ENABLE_1KHZ_OUPUT    false

#  define CONF_CLOCK_XOSC32K_ENABLE_32KHZ_OUTPUT  true

#  define CONF_CLOCK_XOSC32K_ON_DEMAND            true

#  define CONF_CLOCK_XOSC32K_RUN_IN_STANDBY       false

#  define CONF_CLOCK_XOSC32K_FAILURE_DETECTOR_PRE    SYSTEM_CLOCK_XOSC32K_FAILURE_DETECTOR_PRESCALER_1

#  define CONF_CLOCK_XOSC32K_FAILURE_DETECTOR_EVENT_OUTPUT_ENABLE  false

#  define CONF_CLOCK_XOSC32K_FAILURE_DETECTOR_ENABLE  false

#  define CONF_CLOCK_XOSC32K_FAILURE_SWITCH_BACK_ENABLE  false







/* SYSTEM_CLOCK_SOURCE_OSC32K configuration - Internal 32KHz oscillator */

#  define CONF_CLOCK_OSC32K_ENABLE                false

#  define CONF_CLOCK_OSC32K_STARTUP_TIME          SYSTEM_OSC32K_STARTUP_130

#  define CONF_CLOCK_OSC32K_ENABLE_1KHZ_OUTPUT    true

#  define CONF_CLOCK_OSC32K_ENABLE_32KHZ_OUTPUT   true

#  define CONF_CLOCK_OSC32K_ON_DEMAND             true

#  define CONF_CLOCK_OSC32K_RUN_IN_STANDBY        false





/* SYSTEM_CLOCK_SOURCE_DPLL configuration - Digital Phase-Locked Loop */

#  define CONF_CLOCK_DPLL_ENABLE                  false

#  define CONF_CLOCK_DPLL_ON_DEMAND               true

#  define CONF_CLOCK_DPLL_RUN_IN_STANDBY          false

#  define CONF_CLOCK_DPLL_LOCK_BYPASS             false

#  define CONF_CLOCK_DPLL_WAKE_UP_FAST            false

#  define CONF_CLOCK_DPLL_LOW_POWER_ENABLE        false



#  define CONF_CLOCK_DPLL_LOCK_TIME   SYSTEM_CLOCK_SOURCE_DPLL_LOCK_TIME_DEFAULT

#  define CONF_CLOCK_DPLL_REFERENCE_CLOCK   SYSTEM_CLOCK_SOURCE_DPLL_REFERENCE_CLOCK_XOSC32K

#  define CONF_CLOCK_DPLL_FILTER    SYSTEM_CLOCK_SOURCE_DPLL_FILTER_DEFAULT

#  define CONF_CLOCK_DPLL_PRESCALER   SYSTEM_CLOCK_SOURCE_DPLL_DIV_1





#  define CONF_CLOCK_DPLL_REFERENCE_FREQUENCY     32767

#  define CONF_CLOCK_DPLL_REFERENCE_DIVIDER       1

#  define CONF_CLOCK_DPLL_OUTPUT_FREQUENCY        48000000



/* DPLL GCLK reference configuration */

#  define CONF_CLOCK_DPLL_REFERENCE_GCLK_GENERATOR GCLK_GENERATOR_1


    /* DPLL GCLK 32K reference configuration */

#  define CONF_CLOCK_DPLL_REFERENCE_GCLK_32K_GENERATOR GCLK_GENERATOR_1



/* Set this to true to configure the GCLK when running clocks_init. If set to

 * false, none of the GCLK generators will be configured in clocks_init(). */

#  define CONF_CLOCK_CONFIGURE_GCLK               true


/* Configure GCLK generator 0 (Main Clock) */

#  define CONF_CLOCK_GCLK_0_ENABLE                true

#  define CONF_CLOCK_GCLK_0_RUN_IN_STANDBY        false

#  define CONF_CLOCK_GCLK_0_CLOCK_SOURCE          SYSTEM_CLOCK_SOURCE_OSC48M

#  define CONF_CLOCK_GCLK_0_PRESCALER             1

#  define CONF_CLOCK_GCLK_0_OUTPUT_ENABLE         false



/* Configure GCLK generator 1 */

#  define CONF_CLOCK_GCLK_1_ENABLE                true

#  define CONF_CLOCK_GCLK_1_RUN_IN_STANDBY        false

#  define CONF_CLOCK_GCLK_1_CLOCK_SOURCE          SYSTEM_CLOCK_SOURCE_XOSC32K

#  define CONF_CLOCK_GCLK_1_PRESCALER             1

#  define CONF_CLOCK_GCLK_1_OUTPUT_ENABLE         false


/* Configure GCLK generator 2  */

#  define CONF_CLOCK_GCLK_2_ENABLE                false

#  define CONF_CLOCK_GCLK_2_RUN_IN_STANDBY        false

#  define CONF_CLOCK_GCLK_2_CLOCK_SOURCE          SYSTEM_CLOCK_SOURCE_OSC48M

#  define CONF_CLOCK_GCLK_2_PRESCALER             1

#  define CONF_CLOCK_GCLK_2_OUTPUT_ENABLE         false


/* Configure GCLK generator 3 */

#  define CONF_CLOCK_GCLK_3_ENABLE                false

#  define CONF_CLOCK_GCLK_3_RUN_IN_STANDBY        false

#  define CONF_CLOCK_GCLK_3_CLOCK_SOURCE          SYSTEM_CLOCK_SOURCE_OSC48M

#  define CONF_CLOCK_GCLK_3_PRESCALER             1

#  define CONF_CLOCK_GCLK_3_OUTPUT_ENABLE         false


/* Configure GCLK generator 4 */

#  define CONF_CLOCK_GCLK_4_ENABLE                false

#  define CONF_CLOCK_GCLK_4_RUN_IN_STANDBY        false

#  define CONF_CLOCK_GCLK_4_CLOCK_SOURCE          SYSTEM_CLOCK_SOURCE_OSC48M

#  define CONF_CLOCK_GCLK_4_PRESCALER             1

#  define CONF_CLOCK_GCLK_4_OUTPUT_ENABLE         false


/* Configure GCLK generator 5 */

#  define CONF_CLOCK_GCLK_5_ENABLE                false

#  define CONF_CLOCK_GCLK_5_RUN_IN_STANDBY        false

#  define CONF_CLOCK_GCLK_5_CLOCK_SOURCE          SYSTEM_CLOCK_SOURCE_OSC48M

#  define CONF_CLOCK_GCLK_5_PRESCALER             1

#  define CONF_CLOCK_GCLK_5_OUTPUT_ENABLE         false


/* Configure GCLK generator 6 */

#  define CONF_CLOCK_GCLK_6_ENABLE                false

#  define CONF_CLOCK_GCLK_6_RUN_IN_STANDBY        false

#  define CONF_CLOCK_GCLK_6_CLOCK_SOURCE          SYSTEM_CLOCK_SOURCE_OSC48M

#  define CONF_CLOCK_GCLK_6_PRESCALER             1

#  define CONF_CLOCK_GCLK_6_OUTPUT_ENABLE         false


/* Configure GCLK generator 7 */

#  define CONF_CLOCK_GCLK_7_ENABLE                false

#  define CONF_CLOCK_GCLK_7_RUN_IN_STANDBY        false

#  define CONF_CLOCK_GCLK_7_CLOCK_SOURCE          SYSTEM_CLOCK_SOURCE_OSC48M

#  define CONF_CLOCK_GCLK_7_PRESCALER             1

#  define CONF_CLOCK_GCLK_7_OUTPUT_ENABLE         false


/* Configure GCLK generator 8 */

#  define CONF_CLOCK_GCLK_8_ENABLE                false

#  define CONF_CLOCK_GCLK_8_RUN_IN_STANDBY        false

#  define CONF_CLOCK_GCLK_8_CLOCK_SOURCE          SYSTEM_CLOCK_SOURCE_OSC48M

#  define CONF_CLOCK_GCLK_8_PRESCALER             1

#  define CONF_CLOCK_GCLK_8_OUTPUT_ENABLE         false

#endif /* CONF_CLOCKS_H_INCLUDED */