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).