I know you're going to come in here and make up all the excuses you can in the world but the fact that the group of you have been doing the same shit since s9 and it looks like after 3 seasons of failure you have given up on any sort of subtlety.
It doesn't "reserve" it, it just uses up to 20% when it wants to (mostly windows update). Ergo when it isn't updating/downloading shit, 100% of your line is available for application, when it is downloading an update, only 80%.
Regardless, this has almost nothing to do with latency. The only case where usage will affect latency is when it starts throttling, i.e. you're downloading something at max possible speed, and in some rare cases when you're creating too many connections in a short timespan (f.ex. badly configured torrent client).