The built-in ObjectId parseHexString() implementation is somewhat slow, because it calls Integer.parseInt() 12 times. Replacing with hand-coded hex parsing achieves something like a 5x improvement.

Performance test (ran on master and on branch on my M1 Mac):

    public static void main(String[] args) {
        long startTime = System.currentTimeMillis();
        for (int i = 10000000; i < 100000000; i++)

{             new ObjectId("1234567890abcdef" + i);         }

        long endTime = System.currentTimeMillis();

        System.out.println("Time elapsed: " + (endTime - startTime));
    }

On master:

Time elapsed: 19460

On branch:

Time elapsed:  3617