The speed shown on the launcher is the speed at which is downloading data from a server. As part of updating to a new patch, the launcher has to download data and move or reorganize data within files and executables. Often when its displaying very slow speeds its doing the second part. It doesn't have to download any more data but it may have to move things around. I believe if you wait long enough, it should successfully go through? The slower your disk is, the longer this will take. For example having a solid state drive or NVME drive will greatly help speed this step up.
I am hoping that at some point in the future we will represent disk operations better in the UI. Not sure if we have anything immediately planned though.