Serial communication changes my frequency output

[pvx]

I'm using an Arduino Uno to output a frequency to move the speedometer gauge from an information cluster.
Every time i send a serial message (to update the speed and such), the gauge gets moved back a little bit
I presume it's because serial communication uses interrupts.
Is there any way to fix this issue? Here's my code in it's entirety.:

//0.7hz per km/h
//0.1hz per 1000rpm


//A0 - right blinker
#define rBlinker A0
#define lBlinker A1
#define lLights A2
#define ABS A3 
#define lowFuel A4
#define speedo 2
#define tacho 3
#define fuel 10
#define temp 5
long speedodelay;
long tachodelay;
long fueldelay;
int currSpeed;
int currRPM;
int currFuel;
unsigned long previousMicros = 0;
unsigned long previousMicrosT = 0;
bool fuelState;

void setup() {
  // put your setup code here, to run once:
  pinMode(rBlinker, OUTPUT);
  pinMode(A2, OUTPUT);
  pinMode(A3, OUTPUT);
  pinMode(A4, OUTPUT);
  pinMode(A5, OUTPUT);
  pinMode(A1, OUTPUT);
  pinMode(2, OUTPUT);
  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(5, OUTPUT);
  pinMode(6, OUTPUT);
  pinMode(7, OUTPUT);
  pinMode(8, OUTPUT);
  pinMode(9, OUTPUT);
  Serial.begin(115200);
  Serial.setTimeout(50);
}

void loop() {
  // put your main code here, to run repeatedly:
  //digitalWrite(rBlinker, HIGH);
  noInterrupts();
  tachoLoop();
  speedoLoop();
  interrupts();
  if(Serial.available()) {
    String msg = Serial.readString();
    Serial.println(msg.charAt(0));
    switch(msg.charAt(0)) {   
      case 's':
        {
          msg.remove(0, 1);
          
          currSpeed = msg.toInt();
          
          float frequency = currSpeed * 0.7;
          float period = 1 / frequency;
          
          speedodelay = period * 1000000;
        }
        break;

      case 't':
        {
          msg.remove(0, 1);
          currRPM = msg.toInt();
          float frequencyT = currRPM * 0.033;
          float periodT = 1 / frequencyT;
          tachodelay = periodT * 1000000;
        }
        break;

      case 'f':
        {
          msg.remove(0, 1);
          currFuel = msg.toInt();
          float frequencyF = currFuel * 0.04;
          float periodF = 1 / frequencyF;
          fueldelay = periodF * 1000000;
          Serial.println(msg);
        }
        break;
      
      case 'b':
        {
          msg.remove(0, 1);
          int state = msg.charAt(1) == '1' ? 1 : 0;
          if(msg.charAt(0) == 'l') {
            digitalWrite(lBlinker, state);
          }
          else { 
            digitalWrite(rBlinker, state);
          }
          Serial.println(msg.charAt(1));
        }
        break;

    }
  }

  int fuelPWM = map(currFuel, 0, 100, 110, 240);
  
  analogWrite(5, 140); // water temp, 120 is middle, 140 is maax, 100 is 0
  analogWrite(10, fuelPWM); // fuel level, 170 - middle, 240 - full, 110 - empty
}

void speedoLoop() {
  unsigned long currentMicros = micros();
  if(currentMicros - previousMicros >= speedodelay / 2) { 
    previousMicros = currentMicros;
    digitalWrite(speedo, !digitalRead(speedo));
  }
}

void tachoLoop() {
  unsigned long currentMicrosT = micros();
  if(currentMicrosT - previousMicrosT >= tachodelay / 2) {
    previousMicrosT = currentMicrosT;
    digitalWrite(tacho, !digitalRead(tacho));
  }
}

I'm fully aware that it's really ugly, and i'm sorry for that.

[groundFungus]

@pvx

String msg = Serial.readString();

The readString() function blocks. The methods for reading serial data in the serial input basics tutorial do not block and they do not use the potentially problematic String class.

[drmpf]

You don't need to give up the convenience and safety of Strings see
Arduino Software Solutions for a number of simple sketches for reading from Serial (blocking and non-blocking) with their pros and cons.

But you seem to be reading just one char, so try

void loop() {
 .  . . 
 int c = Serial.read(); // note read() returns an int
  if (c != -1) { // read() return -1 if there is nothing to be read. 
    // got a char handle it
    char inChar = c; // need to cast to char c to print it otherwise you get a number instead
    Serial.println(inChar); // echo it if needed
    switch(inChar) {
     case 's':  ///
       // etc
  }

Your minimal use if strings is not a problem but check out my tutorial on Taming Arduino Strings
if you start using a lot of them.