Extremely low performance on node.js 11

Leakage is incredibly slow on my system (with an Intel(R) Core(TM) i7-3610QM CPU).

Take the following minimal example:
```
$ cat leakage.js
const sinon = require('sinon');
const leakage = require('leakage');

it(`test`, function(){
this.timeout( 1000000 );
leakage.iterate( ()=>{} );
});

$ npx mocha leakage.js
(node:7249) ExperimentalWarning: queueMicrotask() is experimental.
✓ test (26544ms)

1 passing (27s)
```

The test time decreases from 26s to ~4.6s if I avoid including sinon. In my real tests, where I require a bunch of other modules besides sinon, testing a no-op function for leaks takes minutes.

Is this the expected behavior?
I don't think so, unless it's due to recent fixes for the latest v8, since several issues mention a suggested timeout of 5000ms.

I'm running node v11.10.0 and mocha 5.2.0 on a Linux 4.20.7 system.