use Date.now() instead of new Date() on responseTime middleware

`Date.now()` is 2X faster than `new Date()`:

```bash
Date.now() x 8,889,858 ops/sec ±1.34% (94 runs sampled)
Date.now() - Date.now() x 4,379,721 ops/sec ±1.04% (92 runs sampled)
Date.now() - startNow x 7,886,918 ops/sec ±2.01% (96 runs sampled)
new Date() x 4,433,139 ops/sec ±1.99% (94 runs sampled)
new Date() - new Date() x 1,508,084 ops/sec ±2.27% (93 runs sampled)
new Date() - start x 2,169,245 ops/sec ±1.74% (90 runs sampled)
```