Once upon a time, one could do exactly that with Postfix. After all, the maildrop is nothing more than a directory that files are written to. The trouble is that one can play all sorts of tricks with a world-writable maildrop directory. (This was long since known even at the time that Postfix was first written.) So on a properly set up system, you won't be able to just write out files.
This almost certainly isn't your bottleneck in the first place. This is, after all, electronic mail that we are talking about, where there's a big thing called "the Internet" that is often the slowest part of the system. I strongly suspect that you haven't analyzed your bottlenecks to see what and where they are, and that your idea that writing directly to a spool file will "speed things up considerably" is based upon hope rather than measurement.
It's fairly easy to eliminate any of the delays in an SMTP Submission dialogue with SMTP pipelining. And, as mentioned in another answer, using Postfix's
sendmail utility — which is little more than a wrapper around
postdrop in this case — is also an option. But saving a second in message submission is likely to be lost in the noise if your real bottleneck is, for example, a dial-up Internet connection.