is-mobile.js

var ITERATIONS = 100000,
    av = navigator.appVersion;

console.time('indexOf')
for (var i = 0; i < ITERATIONS; i++) {
  navigator.appVersion.indexOf("Mobile") > -1
}
console.timeEnd('indexOf')

console.time('regex')
for (var i = 0; i < ITERATIONS; i++) {
  /mobile/i.test(navigator.appVersion)
}
console.timeEnd('regex')


console.time('indexOf-cached');
for (var i = 0; i < ITERATIONS; i++) {
  av.indexOf("Mobile") > -1
}
console.timeEnd('indexOf-cached')

console.time('regex-cached')
for (var i = 0; i < ITERATIONS; i++) {
  /mobile/i.test(av)
}
console.timeEnd('regex-cached')

ndexOf: 470.218ms
regex: 433.497ms
indexOf-cached: 128.361ms
regex-cached: 123.078ms