Probably I have an idea.
cernbox does not accept Ranges in the requests and always return full file content.
Even when it declares Accept-Ranges in the response headers.
JSROOT has workaround - it request complete file content once and then reusing it. Of course, this does not work for large files.
ROOT does not have such workaround and for each small request gets full content again and again.
Therefore it may take very long time to process such file.
Therefore I will not recommend to use cernbox for such applications - before problem will be fixed.