Tachometer. Resolution vs speed issues.

Hello everyone.

I am more of a hardware dude, so software always makes me stay up all nights while trying to understand why it doesn't work and why DOES it work.
Anyway, below is the code I am using. It works, kind of, but there is a problem.

If I decrease delay, resolution goes down pretty bad, with 300ms it is 100RPM. If I Increase, of course it gets slow, and that is not ok in my application. As I am a total noob, I hope someone here might give me advice, what can I possibly do.
Regards, Arch.

 

volatile unsigned int rpmcount;
float rpm;
float timeold;

const int carbswitch = P2_2; //switch from carburator, when accelerator is not pushed, switch is connected to GND. When accelerator is getting floored, disconnects.
const int outputswitch = P2_4;  //enables/disables gas valve, HIGH at >1400 RPM and carbswitch HIGH.
const int shiftlight = P2_5;  //enabled when RPM > 5000.
const int engineok = P2_3;  //enabled when RPM > 500.
int carbswitchstate = LOW;
int valvestate = LOW;
int mymilis = 300;
String ds;


#include <LiquidCrystal.h>

LiquidCrystal lcd(P1_2, P1_1, P1_4, P1_5, P1_6, P1_7);

void rpm_fun()
 {
   rpmcount++;
 }
//INPUT from ignition coil's switching GND is wired to P1_3, with protection from high voltage spikes.
void setup()
 {
   lcd.begin(16, 2);
   attachInterrupt(P1_3, rpm_fun, FALLING);
   rpmcount = 0.0;
   rpm = 0.0;
   timeold = 0.0;
   pinMode (outputswitch, OUTPUT);
   pinMode (shiftlight, OUTPUT);
   pinMode (engineok, OUTPUT);
   pinMode(carbswitch, INPUT_PULLUP);
 }

 void loop()
 {
   if(millis()-timeold>=mymilis){
   detachInterrupt(P1_3);

  const float freqHz = ((float) rpmcount / (float) (millis() - timeold)) * 1000.0;
  const float rpm = freqHz * 30.0;
   rpmcount = 0;

   carbswitchstate = digitalRead(carbswitch);
   if (rpm >= 1400.0 && carbswitchstate == HIGH) {
    digitalWrite(outputswitch, HIGH);
   }
   else {
    digitalWrite(outputswitch, LOW);
   }
  valvestate = digitalRead(outputswitch);
   if (valvestate == HIGH) {
    ds = ("ON");
   }
   else {
    ds = ("OFF");
   }
   if (rpm >= 5000.0) {
    digitalWrite(shiftlight, HIGH);
   }
   else {
    digitalWrite(shiftlight, LOW);
   }
   if (rpm >= 500.0) {
    digitalWrite(engineok, HIGH);
   }
   else {
    digitalWrite(engineok, LOW);
   }

  
   lcd.clear();
   lcd.print("RPM=");
   lcd.print(rpm);
   lcd.setCursor(0, 1);
   lcd.print ("ECON=");
   lcd.print(ds);
   
   attachInterrupt(P1_3, rpm_fun, FALLING);
   
  timeold = millis();
   }
  }