The very nature of an interrupt dictates that it interrupts the regular execution of the processor, switching to the interrupt handler instead. Any microsecond that we spend in the interrupt handler code is a microsecond during which we aren't running the other routines or handling other interrupts.
To prevent any issues arising from this, interrupt handlers (ISRs) should be kept as short as possible, ideally merely updating a single value in a quick and safe manner before ending the ISR and resuming normal operation.
