open System
let ranStr n = 
    let r = Random()
    let chars = Array.concat([[|'a' .. 'z'|];[|'A' .. 'Z'|];[|'0' .. '9'|]])
    let sz = Array.length chars in
    String(Array.init n (fun _ -> chars.[r.Next sz]))

// > ranStr 100_000_000;;
// Real: 00:00:01.555, CPU: 00:00:01.575, GC gen0: 1, gen1: 1