Why is it so big?
Unlike your VB.Net program, which needs a few megabytes of .Net framework to run, while the C++ program will run all alone, using not a single DLL besides the standard windows DLLs.
If this is alredy release then the 400 kB come from the statically linked C++ standard library you're using. Your code alone would be about 1-2 kB large. Add to that standard exe bloating and you'll end up with a 15-20 kB app.
The VB.Net app on the other hand needs 50 kB only for your code and .Net exe bloating...


Why is it so slow?
Because the programming language doesn't make a f***ing difference when all you're doing is accessing a 5MB file on the hard disk. The other computer probably has a faster hard disk, or maybe the file access is somehow more efficient (larger buffer or something), but it hasn't got anything to do with the language.