Just a simple way to see how slow your code gets when using eval().

evalRuntimeAverage.js

function doSomething( str, a ) {
  eval( str );
  console.log(a);
}

function getRandomInt( min, max ) {
  min = Math.ceil(min);
  max = Math.floor(max);
  return Math.floor( Math.random() * ( max - min ) ) + min;
}

var average = [], t0 = 0, t1 = 0;

for ( let i = 0; i < 10; i++ ) {
  t0 = performance.now();
  doSomething(`a=${ getRandomInt( 1, 1000 ) };`, ( getRandomInt(1, 1000) * i ));
  t1 = performance.now();
  
  average.push( t1 - t0 );
}

function getAverage(params) {
  let totaledAverage = 0;
  
  params.forEach(param => {
    totaledAverage += param;
  });

  return ( totaledAverage / params.length );
}

console.log('Average: ' + getAverage(average));


// eval average: 0.21049999999959254
// non-eval average: 0.12049999999999272