MSP432E411Y-BGAEVM: My UART6 RX EOT interrupt is too slow

I did what I believe is the same thing, and I get timer_before = 119999972, timer_after = 118899120, for a difference of 1100852. I left the timer prescaler=1, so it was counting at 120MHz.  My calculator says 1100852/120MHz = 0.009174 seconds i.e. about 9.2ms.

[Edit: On re-reading I realized that what I called timer_after corresponds with timer_int in your case.]

My scope indicates a delay of about 100us between the final Tx rising edge (stop bits) and the falling edge of the enable. (I don't know whether EOT is defined to trigger after the final data bit or after the stop bit(s).)

Just for clarity, here's what I was working with. Maybe you can see what's different.

///
#define BMC     1
#define TIMERS  1
#include <stdio.h>
#include <stdlib.h>
#include "ti/devices/msp432e4/driverlib/driverlib.h"
#include <ti/devices/msp432e4/driverlib/interrupt.h>
#include <ti/devices/msp432e4/driverlib/uart.h>
#include <ti/drivers/uart/UARTMSP432E4.h>
#include <ti/devices/msp432e4/driverlib/gpio.h>
#include <ti/devices/msp432e4/driverlib/sysctl.h>
#include <ti/devices/msp432e4/driverlib/pin_map.h>
#if !BMC
#include <ti/devices/msp432e4/inc/msp432e411y.h>
#endif

volatile unsigned char buffer[256] = { { 0 } };
volatile unsigned int buffer_position = 0;
#if BMC // BMC
#define ENBIT GPIO_PIN_0    // No PH4 on E401Y
#else
#define ENBIT GPIO_PIN_4
#endif
#if TIMERS
volatile uint32_t time_before, time_after;
#endif
/* Incoming byte at UART6 interrupt handler. */
void UART6_IRQHandler(void) {
    uint32_t ui32Status;
#if TIMERS
    time_after = TimerValueGet(TIMER0_BASE, TIMER_A);
#endif // TIMERS
    // Get the interrupt status.
    ui32Status = UARTIntStatus(UART6_BASE, true);

    // Clear the asserted interrupts.
    UARTIntClear(UART6_BASE, ui32Status);

    if (ui32Status & UART_INT_TX) { // end-of-transmission interrupt
        GPIOPinWrite(GPIO_PORTH_BASE, ENBIT, 0x00);
    }
    while (UARTCharsAvail(UART6_BASE)) {
        buffer[buffer_position++] = UARTCharGet(UART6_BASE);
    }
}

int main(void) {
    uint32_t ui32SysClock;
    volatile uint32_t ui32Loop;

    // Run from the PLL at 120 MHz.
    ui32SysClock = SysCtlClockFreqSet((SYSCTL_XTAL_25MHZ | SYSCTL_OSC_MAIN | SYSCTL_USE_PLL | SYSCTL_CFG_VCO_480),
                                      120000000);

#if BMC // BMC
    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOP);
    while (!SysCtlPeripheralReady(SYSCTL_PERIPH_GPIOP)) {
    }
#endif // BMC
    // Initialise UART 6.
    SysCtlPeripheralEnable(SYSCTL_PERIPH_UART6);
    IntMasterEnable();

    GPIOPinConfigure(GPIO_PP0_U6RX);
    GPIOPinConfigure(GPIO_PP1_U6TX);
    GPIOPinTypeUART(GPIO_PORTP_BASE, GPIO_PIN_0 | GPIO_PIN_1);
    UARTConfigSetExpClk(UART6_BASE, ui32SysClock, 9600,
                        (UART_CONFIG_WLEN_8 | UART_CONFIG_STOP_ONE | UART_CONFIG_PAR_EVEN));
    IntEnable(INT_UART6);
    UARTIntEnable(UART6_BASE, UART_INT_RX | UART_INT_TX);
    UARTTxIntModeSet(UART6_BASE, UART_TXINT_MODE_EOT);

    // Initialise GPIO pin.
    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOH);
    while (!SysCtlPeripheralReady(SYSCTL_PERIPH_GPIOH)) {
    }
    GPIOPinTypeGPIOOutput(GPIO_PORTH_BASE, ENBIT);
    // Send message.
    uint8_t data[8] = { 0x01, 0x04, 0x00, 0x00, 0x00, 0x05, 0x30, 0x09 };
#if TIMERS
    MAP_SysCtlPeripheralEnable(SYSCTL_PERIPH_TIMER0);
    MAP_TimerConfigure(TIMER0_BASE, TIMER_CFG_PERIODIC);
    MAP_TimerLoadSet(TIMER0_BASE, TIMER_A, ui32SysClock); // 1 second for no good reason
    MAP_TimerEnable(TIMER0_BASE, TIMER_A);
    time_before = TimerValueGet(TIMER0_BASE, TIMER_A);
#endif // TIMERS
    GPIOPinWrite(GPIO_PORTH_BASE, ENBIT, 0xFF);
    int i;
    for (i = 0; i < 8; ++i) {
        UARTCharPutNonBlocking(UART6_BASE, data[i]);
    }

    while (1) {
    }
}

[Edit: Fixed typo.]