Ah, okay, so when you said “would it bring any benefit in speed to call julia from R and call the R function from julia” in your OP, you actually meant just using Julia to call the function you already have in R to speed it up. To that question the answer is no, I answered a similar question here a while ago with a link to a nice summary by Stefan of why Julia can’t magically speed up code written in other languages.
Now I don’t mean to sound condescending in any way so don’t get me wrong, but I think the “some adjustment” really wouldn’t be very hard to write in Julia (trust me I’m one of the more mediocre programmers on this forum!)
If you can get together five test cases and knock up an initial version of the function I’m sure people on here would be happy to help you get this over the line quite quickly.
Whether it makes sense to spend any effort on this of course really depends on how often you’ll need this function - when you say you’re quite close to finishing this project it might not be worth spending any time on this, but if you have to call it a few dozen more times before you finish your project then reducing the time from 20 minutes to 20 seconds might be well worth it!