Historical DOS question

[Edenyard]

   Here's a history question!! Please can anyone tell me what the
equivalent to MS-DOS 5.0 was in IBM PC-DOS or DR-DOS version numbers? I
have an old 486 that ran well on MS-DOS 5.0 but seems noticeably slower
on IBM PC-DOS 7, so I'd like to find an old(er) IBM PC-DOS that would
run on the machine in a similar way.

   Are there any URLs for comparisons, historical development, etc., of
the various DOSes?

   Any clues gratefully received - thanks!

      Gerald.