I've heard of the slow context menu J-Mac - had it to a certain extent on my last computer (the longer the install the worse it got..) but never
that bad
Would drive me mad alright

Re changing filenames for a series of files, a good trick in DOpus is:-
Select file
Press F2
Name part gets selected - rename
DONT press return -
Press down arrow and next file is selected, editable and ready for rename
and so on down the list
I presume you know that when filename is editable, that right arrow brings you to end of name but before the dot, for adding suffix:
Filename|.ext
Left arrow brings you to beginning of name
*********
That problem with images is extremely odd - all images ??
Have you reported it as a bug?
What's your default viewer? - have you changed it lately ?
(I ask cause I had problems after setting ExifPro as default viewer

- but not like that problem..)