Re: "The more checks, tests and privilege validaions you put in place...."
> DOS was very, very fast.
No, it wasn't. Display calls to MS-DOS were very, very slow. Display calls to BIOS were passable fast. If you wanted very fast display you bypassed both and did direct screen writes, just like most professional software did.
MS-DOS was also very slow on file access, in particular on large data files that required random access, due to the way FAT worked. Large ISAM files were particularly slow compared to other systems because in order to access a particular position within the file the OS, for each access, had to start at the directory entry and follow down the FAT table until it found the appropriate cluster. That is why defragging was required, by bringing all the FAT entries for a file together it reduced the number of data blocks required to be read. iNode systems, for example, could access any part of a large data file with many fewer block reads.
DR-DOS had a feature that was not available in standard MS-DOS and that was the cluster size could be specified when a partition was formatted (other utilities could also do this). On a particular partition size MS-DOS would only give, say, 2KB cluster size. Using DR-DOS to give an 8KB cluster size would give an improvement of 3x for random access to a 1Megabyte ISAM data file with no other change. This was solely because there were 4x fewer FAT entries to access.
The only reason that MS-DOS was perceived as being 'fast' was because it could be bypassed by the programs and didn't get in way.
Biting the hand that feeds IT
