Ran with wrk http://localhost:<port>/_matrix/media/r0/<route> -c64 -t8 -d1m,
for a 1 minute benchmark across 8 threads, with 64 requests open at a time.


**synapse-media-proxy refactor**
```
Running 1m test @ download/pixie.town/Nrp1oyUgHirgm7gDSlDvHutM
  8 threads and 64 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   137.02ms   51.40ms 826.66ms   98.06%
    Req/Sec    60.67     11.64    90.00     80.03%
  28740 requests in 1.00m, 170.97GB read
Requests/sec:    478.23
Transfer/sec:    2.84GB

Running 1m test @ thumbnail/pixie.town/OjN4zjsGXzSZBkLBsHh5REJi?height=128&width=128
  8 threads and 64 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency    12.99ms   16.72ms 371.69ms   99.23%
    Req/Sec   682.69     65.13   808.00     88.66%
  324549 requests in 1.00m, 1.36GB read
Requests/sec:   5405.62
Transfer/sec:   23.18MB
```

**synapse-media-proxy original**
```
Running 1m test @ download/pixie.town/Nrp1oyUgHirgm7gDSlDvHutM
  8 threads and 64 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   398.98ms   47.92ms 874.86ms   96.83%
    Req/Sec    25.00     17.51    70.00     74.47%
  9612 requests in 1.00m, 57.22GB read
Requests/sec:    160.01
Transfer/sec:    0.95GB

Running 1m test @ thumbnail/pixie.town/OjN4zjsGXzSZBkLBsHh5REJi?height=128&width=128
  8 threads and 64 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     1.47s    72.76ms   2.00s    96.48%
    Req/Sec    14.74     12.64    70.00     54.84%
  2588 requests in 1.00m, 8.31MB read
  Socket errors: connect 0, read 0, write 0, timeout 3
Requests/sec:    43.07
Transfer/sec:    141.64KB
```

**Synapse**
```
Running 1m test @ download/pixie.town/Nrp1oyUgHirgm7gDSlDvHutM
  8 threads and 64 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     1.06s    61.43ms   1.33s    81.81%
    Req/Sec    14.13     14.20    70.00     81.42%
  3585 requests in 1.00m, 21.66GB read
Requests/sec:     59.71
Transfer/sec:    369.38MB

Running 1m test @ thumbnail/pixie.town/OjN4zjsGXzSZBkLBsHh5REJi?height=128&width=128
  8 threads and 64 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   206.96ms  101.69ms   1.38s    97.21%
    Req/Sec    41.42     16.39    80.00     61.90%
  19278 requests in 1.00m, 15.42MB read
  Non-2xx or 3xx responses: 19278
Requests/sec:    321.01
Transfer/sec:    263.01KB
```
(note a lot of those requests returned non-2xx/3xx responses, failing to thumbnail)