Why is partial loading of matfile "inefficient" for -v7, but still faster?

When executing the following code, I get a warning that partial loading the -v7 *.mat file is "inefficient" and am told to use -v7.3 instead. However, when I test the execution times of each, it is actually the -v7 file which is faster. Why is this? To what "inefficiencies" does the warning refer?

>> whos

Name        Size              Bytes  Class

b           1x1                   8  double              
c         641x143            366652  uint32              
d           1x143             19296  cell                
e           1x1                 128  cell

>> save('G:\deletme0.mat','b','c','d','e')

>> save('G:\deletme1.mat','b','c','d','e','-v7.3')

>> save('G:\deletme2.mat','b','c','d','e','-v7.3','-nocompression')

>> tic; m = matfile('G:\deletme0.mat'); q = m.c(1:10,1:50); toc

Warning: The file 'G:\deletme0.mat' was saved in a format that does not support partial loading. Temporarily loading variable 'c' into memory. To use partial loading efficiently, save MAT-files with the -v7.3 flag. > In matlab.io.MatFile/inefficientPartialLoad (line 144) In matlab.io.MatFile/subsref (line 462)

Elapsed time is 0.009319 seconds.

>> tic; m = matfile('G:\deletme1.mat'); q = m.c(1:10,1:50); toc

Elapsed time is 0.013688 seconds.

>> tic; m = matfile('G:\deletme2.mat'); q = m.c(1:10,1:50); toc

Elapsed time is 0.013699 seconds.

Answer 1

[Philip Borghesani]

V7 matfiles are faster in general and can be much faster if many small structures are saved. When working with large files containing large variables V7.3 files can be partially loaded reducing time and memory while the entire V7 file must be loaded to extract any variable.

You might see different results if the sizes of c and d were in the millions.

[Joseph Hall]

Thank you. Below shows a quick test which confirms your suggestion that -v7.3 files can be more efficient/faster than -v7 when partial-loading a larger size matrix.

>> a = zeros(1e6,50);
>> whos a
  Name            Size                Bytes  Class     Attributes

    a         1000000x50            400000000  double              

>> save('G:\deletme0.mat','a','-v7')
>> save('G:\deletme1.mat','a','-v7.3')
>> save('G:\deletme2.mat','a','-v7.3','-nocompression')
>> tic; m = matfile('G:\deletme0.mat'); q = m.a(1:10,1:50); toc
Warning: The file 'G:\deletme0.mat' was saved in a format that does not
support partial loading. Temporarily loading variable 'a' into memory. To use
partial loading efficiently, save MAT-files with the -v7.3 flag. 
> In matlab.io.MatFile/inefficientPartialLoad (line 144)
  In matlab.io.MatFile/subsref (line 462) 
Elapsed time is 0.228414 seconds.
>> tic; m = matfile('G:\deletme1.mat'); q = m.a(1:10,1:50); toc
Elapsed time is 0.006759 seconds.
>> tic; m = matfile('G:\deletme2.mat'); q = m.a(1:10,1:50); toc
Elapsed time is 0.036743 seconds.