Context

• node version: 8.9.4
• joi version: 13.1.1
• environment (node, browser): NodeJS
• used with (hapi, standalone, ...): standalone
• any other relevant information: -

What are you trying to achieve or the steps to reproduce ?

const Joi = require('joi');

const list = [];
for (let i = 10000; i < 50000; i++) {
  list.push(i.toString());
}

const filteredList = list.filter(x => !['12345', '23456', '34567', '456789'].includes(x));

console.time();
Joi.object().keys({
  foo: Joi.string().valid(filteredList),
});
console.timeEnd();

Which result you had ?

It works very slow (25385.524ms - almost half a minute).

What did you expect ?

It should work much faster (shorter than 1 second).