Just to touch on a few points here.
RM and all the applications that we publish are written in groovy, and they are written the exact same way as applications written by users, sure there are one or two internal shortcut methods that for security reasons only work with system applications, but that's a very very short list.
RM is big, given the options that it presents, it's going to be big, there's no getting around that, with flexibility comes complexity.
Personally I only have two custom apps that I run (one to manage dimmer levels, the other connects my hubs (pre HubConnect)), having said that my rules are simple and seldom exceed one if/else block.
So the point here is if you have a significantly sized Rule and it's correctly written and you aren't happy with the execution speed, sure, creating a dedicated application to replace it is going to run faster.
In almost every case a well written specific dedicated app is always going to perform faster than a larger more complex swiss army multipurpose app...
Having said that not everyone wants to write or needs to write code, but with Hubitat you have the option to create exactly what you want if so desired...