test.js

var start = (new Date()).getTime();
var testArray = ["j","a","b"];

for( var i = 0; i < 5000000; i++ ) {
    if( Array.isArray(testArray) ) {};
}

console.log( (new Date()).getTime() - start + " ms" );
start = (new Date()).getTime();

for( var i = 0; i < 5000000; i++ ) {
    if( testArray instanceof Array ) {};
}

console.log( (new Date()).getTime() - start + " ms" );

/*Result
"C:\Program Files (x86)\nodejs\node.exe" test.js
37 ms
14 ms
    */