A problem with Explorer, that it also shares with macOS Finder[1], is that they are very much legacy applications with features piled on top, and Explorer was never expected to be used for heavy I/O work and tends to do things the slower way possible, including doing things in ways that are optimized for "random first time user of windows 95 who will have maybe 50 files in a folder"

[1] Finder has parts that show continued use of code written for MacOS 9 :V