The admin router's /service/ location strips the Accept-Encoding header from requests before proxying them to a backend: https://github.com/dcos/dcos/blob/86a6934657c3d1c22f77d3cd21e112f940988043/packages/adminrouter/extra/src/includes/service-location.common.conf#L1
This prevents a service from compressing its responses, which may have a noticeable impact. An example is the dcos-monitoring package's Grafana UI, which sends a lot of data between the browser and the server. Gzipping this data makes Grafana much quicker to load, and makes certain features more responsive, such as metric name autocompletion.
These screenshots show requests to dcos-monitoring's Overview dashboard (cluster is in AWS, client is on SF office wifi, browser cache is disabled). The first request is with the adminrouter from master, and the second request is with an adminrouter that has the line linked above commented out. Gzip reduces the total page size from 5 MB to 1.6 MB, and the page load time from 1.18 s to 972 ms.
(The Content-Encoding of responses isn't in these screenshots, but I confirmed that it was not present in the first example, and gzip in the second example.)