@Hellomatey001 obviously everything I say is either opinions or experiences so your milage may vary!
But what you'll usually see is a spike in earnings after release (per graph below, this is my book that released this month) and then things will tail off.
After the 30 day mark is where you will lose your spot in the "hot new releases" for each category you've selected, and many authors call this the 30 day cliff.
However some books are what we call "sticky" in that they've achieved a good enough rank to make it into the main category (not the HNR) so their cliff is usually longer.
After the cliff is when things generally tend to settle down (at least that's been my experience). Book 1 published in April makes me a steady 2 figures a day. Book 2 (June) day in day out gives me between £4-7. Doesn't sound like much but £7 x 30 days is still £200. And it all adds up. Book 3 has been very sticky and I've not had a day below 3 figures yet. But I'm not quite sure if that's fallen off the cliffs edge yet.
So you don't have to write fast. There are 2 reasons people do write fast
▪️the first being basic maths. If you have 20 books each making roughly £7 a day, that's an income of £50k a year, which I'm sure most folks in the UK (except maybe London) would call a pretty good income -- especially because you're still making money even when you have a sick day, or a holiday, or a school play.
▪️the second reason to write fast is that nothing sells the last book like writing the next book. Now I'm getting more reviews on goodreads it's quite common for them to say stuff like "this was a new author for me but I love her writing style and I'm off to devour her other works". If you don't want to be constantly promoting ALL of your books ALL the time then the best way to build residual income is to give those readers someplace else to go after they enjoy your book.
So there are good reasons to write fast, but that doesn't mean you can't be successful if you don't write fast. You're journey to success might be slower than someone putting out 2 books a month, or it might be faster, there are literally 1000 variables.
What I can tell you though is that I thought I was a slow writer, I did a couple of short stories to get a feel for publishing and I remember when 5k felt like an absolute slog and took me over a week. Now I can do 5k in a day relatively easy. My first book took 6 weeks around a full time job, and my 5th book (70k) took 12 days start to finish, in the summer holidays, with 4 kids at home.
It's kinda like running, the more you do it the further you can go. You'll get to the point where you stop caring and agonising about the marks on the page, the commas, etc and you'll see it's just a way of putting a story into someones head.
Sorry this was loooooong 