I would have thought the builtin Math.abs was fastest - the Math functions generally just call functions in the underlying OS, making them fast (instead of using bytecode which would be slower but guarantee they were the same).
So there may not be a faster way of doing it yourself. An inline tenary operation which avoids a function call is the only one I can imagine being better (though do test them). Or replace the abs altogether. If e.g. your code looks like this
if (Math.abs(x) < 2) {
// ...
}
Then this is probably faster
if (x * x < 4) {
// ...
}