Here's one way you might find such a function.
Mathematica generally has descriptive names even for undocumented internal functions, so you can often find useful things by searching for appropriate substrings in symbol names. For example:
(* do a PNG import to make sure relevant functions are loaded *)
(* search for symbols containing "PNG" *)
One of the symbols is
Image`ImportExportDump`ImageReadPNG which sounds like it might be useful. Check if we can see the definition:
In this case there is nothing useful here.
To see if (and how) the function is used during a PNG import, use
TraceInternal -> True]
So it looks like the function just takes the filename as an argument. Next you can test it and time it to find out if it's any better than simply using
This is the sort of approach you need to take to uncover undocumented functions - there is inevitably an element of trial and error and often you will find nothing useful at all. Of course you should always save your work before this kind of experimentation.