REALLY, REALLY slow deleting large batch of files

This is an interesting suggestion as the MCH may be trying to sync with the mobile device(s) and the processes slowed when it is unable to do so. When the MCH was first introduced and WD support was more talkative, there used to be good discussions on how the mechanics worked under the hood for MCH and it was suggested that the slow speed were due to the use of the free and open source Syncthing in MCH. In Feb 2018, it was explained by WD support this way:

So the suggestions for the case use for MCH would be not to try to sync frequently updated and editable files such as Excel, Word and email client database files such as those from Thunderbird.