Change timeout of `Serial.parseInt()`?
Don't use Serial.parseInt(). It is very very wrong. The whole idea behind how it operates is completely wrong and teaches bad practices and methods.

Part of your way of speeding it up is half-way to doing it properly.

You require a delimiter to say "this is the end of the number". Relying on time in a system that is inherently asynchronous is just plain daft. You have chosen to add a space. A better choice is to use a proper line ending "\n".

Then you should either read the serial character-by-character and parse it as it arrives until you find the EOL character, or read it character by character into a string (NOT a String, a "string" - a C string - a character array terminated by a NULL character), and then use atoi() etc to convert it to the numerical format you want.

Many of the methods that Arduino provide are not suitable for professional use and should be avoided at all cost lest they teach you the completely wrong way of doing things.