In Catherby while lobster fishing, when you finish an inventory it takes about 15 seconds to start running to the bank. I'm guessing that's to simulate being an afk fisher?
I was under the impression that you're saying the bot doesn't even fish at all?
To answer your question though... The longer you use the bot, the faster it will get. All of my bots use RuneMate Default Web for traversal, which allows the bot to generate unique paths for each player during their session when trying to get from point A to point B. These paths are stored during the session for future use, and therefore the bot will increasingly get smarter in how it should traverse while allowing each user to be unique in their pathing.
These paths do not carry over between sessions, so the first couple of minutes that one of my bots is ran it is building your session's profile.
TLDR, the bot will get faster the longer you use it.
EDIT And also, something worth mentioning is that after your character starts fishing there's a delay created until the next interaction. This is playersensed allowing each player to be unique, but may also cause what you're describing... in which case is intentional.