By default all interrupts including SysTick used by FreeRTOS itself are set to lowest prio.
So they don’t add jitter to the user application when using nested interrupts, which is the default for all MCUs supporting it like Cortex-M3/M4F/7.
Interrupt response might be influenced by FreeRTOS and application code for ISRs using FreeRTOS API due to (very short) critical sections needed by FreeRTOS code but this is usually not a showstopper.
However, if you need very hard real-time behavior you can use ISRs running outside the scope of FreeRTOS (at higher prio) with the appropriate FreeRTOS configuration.
See e.g. this thread and the official docs https://www.freertos.org/RTOS-Cortex-M3-M4.html for the details.