Consecutive requests are getting slower and slower

see [reproduction repo](https://github.com/Allsimon/example-servant-minimal)

```
# copy pastable reproduction steps
git clone https://github.com/Allsimon/example-servant-minimal && cd example-servant-minimal
stack build && stack exec example-servant-minimal

# in another shell
./reproduce.sh
```

This is going to send batches of 1000 requests to a really simple endpoint: requests are taking more and more time.
```
➜ example-servant-minimal git:(master) ✗ ./reproduce.sh
Time per request: 0.281 [ms] (mean, across all concurrent requests)
Time per request: 0.504 [ms] (mean, across all concurrent requests)
Time per request: 0.886 [ms] (mean, across all concurrent requests)
Time per request: 1.034 [ms] (mean, across all concurrent requests)
Time per request: 1.416 [ms] (mean, across all concurrent requests)
Time per request: 2.144 [ms] (mean, across all concurrent requests)
Time per request: 2.041 [ms] (mean, across all concurrent requests)
Time per request: 2.306 [ms] (mean, across all concurrent requests)
Time per request: 2.663 [ms] (mean, across all concurrent requests)
Time per request: 2.905 [ms] (mean, across all concurrent requests)
Time per request: 3.134 [ms] (mean, across all concurrent requests)
```