I have been told to write large applications in several different files. They say it will run faster. What makes it run faster?
Also does a multifile application ACTUALLY run faster than a singlefile one
In C, there is no reason to assume that a multi-source-file application will run faster, and several reasons why it might be slightly slower. Use of multiple files is for the convenience of the developers.
There are even some build systems which let you write multiple files which are combined into a single huge file before being passed to the compiler. This may be called "unity" or "amalgamation" builds.
The main reason is inlining: replacing a call to a function with a copy of the function. C will not usually inline functions from different source files unless the linker is configured to do so. This feature is called "link time optimisation" in GCC.
Other commentators have mentioned the idea of not loading unused bits of the program. However, the function grouping provided by the source files may not be preserved in the executable; unless you're using an "overlay" system (mostly obsolete), or you have separated some features into "plugins.”
On Linux and other systems with virtual memory, the operating system can unload unused parts of the program on its own.
There are also some minor benefits possible from things like string constant deduplication - again, you can have the linker handle this, but it may not be on by default.
Build speed issues are worth considering. If you split a program into N files, where N might be in the 100-10,000 range, then compiling one file is usually much quicker than compiling all N files. However in some cases gathering all N files into one huge file is faster than compiling them all separately - provided it doesn't crash the compiler. This tends to be more important in C++ where compile times can be much longer.
In practice, developers split source files because it makes them easier to work with. Opinions vary but 1,000 lines is a good guideline number to start considering splitting a file.