This is consequence of issue #236439.
On a very slow FS the FileObject.refresh() and reading from a stream can take a considerable time. When document is write-locked during this time the AWT may be waiting on document's readlock so the IDE looks like frozen.
The read and reload operations in o.o.text.DocumentOpenClose should be modified to
1. Perform FO.refresh()
2. Read file content into a String and remember time when the file content was read.
3. Perform Document.runAtomic() and insert the String into the document.
A similar approach should be made for reload but in addition there will be a possibility that the user may type several characters during the reload operations so such situation will have to be handled accordingly.
Issue 236761 (showing dialog for incorrect charset during load) should be possibly be resolved together with this issue.