LFGabel

We have made fixes/improvements to this area (Slow text entry with large text files) of the program in the latest customer beta. If you would like to try these changes the beta software is available in the forum posts listed below.
The latest beta builds are downloadable from links at the top of each of these beta forum posts.
Affinity Publisher 1.8.0.556 for Windows Affinity Publisher 1.8.0.556 for Mac These betas install parallel, next to the release version (they do not overwrite your release) and so the fixes can be tried in the beta without affecting your normal workflow in the release version.
Once these programs have been through a full beta process the change will be released in a future free 1.8.0 update/patch to all customers.

Thanks for the thorough answer Tegwyn, I really appreciate it! I'm not scared off. For this particular project, I nevertheless chose to go back to InDesign, but mostly as I still have overlapping licences. I'm a proponent for Publisher, and often endorse it at work, where me and my colleagues mostly use it for scientific posters, pamphlets and questionnaires. I will give it another try next time I work with relatively short books such as theses. My current project is a clear deviation by being over 800 pages long. If I run into troubles at a late stage, much work will have been in vain.

Overall, I think you should be fine. Details about my experience:
The only image-based slowdown I've run into is when I used an image directly rather than placing it in a frame, and I set its wrapping to tight. The automatic wrapping outline had a very large number of nodes, and that chapter became sluggish. (I believe they have plans to fix this.) Things improved when I deleted most of the nodes to simplify the wrap outline. When I've put the images in frames and made the wrap outline myself, it's been fine.
Every once in awhile, a change will take a long time to compute, and certain things are likely to cause crashes for me, such as editing global swatches (fixed in beta) or undoing and redoing table stuff. Saving takes awhile on my large project. (Also fixed in the beta version: be careful about placing docx files with tables and changing tables between pinned and unpinned. I've had that (apparently) corrupt my files before. If you're better than I am about saving new versions of your file frequently, then you won't have to redo as much if this happens. I'll be switching to the beta to keep this from happening again.)
Publisher is generally a little laggy, but workable for me. I'm on a mid-range machine, so it might just be my computer.
It's worth noting that most users don't seem to be encountering the issues I've mentioned. I've got a wimpier computer and a complicated document, so I don't want to scare you off. Publisher is a very cool program.

Angalanse, you might want to try breaking up your document into smaller blocks of text (chapters) until they isolate and squash this problem. I'll be trying that with my next book but I won't be doing that for a few months. InDesign is not an option for me any longer.
With thanks,
Lee

@Angalanse For what it's worth, after my post, I ended up breaking it up into a different text flow for each chapter, and it helped enough for me to turn baseline grid back on. I was worried that splitting it up would break hyperlinks, but it did not. I went to each chapter break, selected the start of that chapter plus Ctrl+Shift+End (to select from there to the end of the text flow), cut, disconnected the text frame from the previous one, then pasted again. You might find this works for you if you have any spots where there is a clean split between pages (like a chapter title).

Hi @Tegwyn,
As I recall, I did have the baseline grid on. Not displayed, but enabled. Turning it off is not an option for me because snapping to baseline and having lines line up between spreads increases the reader experience. 
With thanks,
Lee

Hi @Tegwyn,
As I recall, I did have the baseline grid on. Not displayed, but enabled. Turning it off is not an option for me because snapping to baseline and having lines line up between spreads increases the reader experience. 
With thanks,
Lee

So I dug a little deeper and realized I had two versions of the ZapfHumanist font installed: One a True Type font from Corel (like Lagarto suggested) and one from a Bitstream font collection which identified as an Open Type font.
I removed the Bitstream Open Type font, reloaded Publisher, and all is working as it should now.
Pauls, I can upload the font, publisher doc and a PDF if you still require it. Just let me know.
Thanks for the help, but sorry for wasting everyone's time.
With thanks, Lee

Yes, same problem here on a 250 pages document with only vectors for decoration on the master pages. The app is slow and the file is huge for no reason (when compared to using other apps).

(...)

I have begin using Publisher to layout novels. However, speed issues are really slowing me down, especially on text that is flowing over hundreds of pages.
Other aspects of the application are slow as well, not just text entry. Selecting fonts and sizes, applying styles, and changing visual options like showing/hiding baseline grid is so slow, a force quit is required.
The below video shows a text entry example with a file that has virtually no graphics. It is mostly text, 503 pages long. To show the difference in performance, I first type into a single non-linked text frame. Everything works as expected. However, editing text in a linked text frame, the lag between characters appearing onscreen is almost ten seconds, per character. In fact, during that time Windows thinks the application has become unresponsive. It is really difficult to work like this. 
Any ideas? Is this a bug or just me? I hope it's a setting on my computer I can change.
Demonstration of slow performance (Youtube Unlisted video)
Affinity Publisher 1.7.2
Windows 10 Home
Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz
16GB Memory
2TB HDD / 128GB SSD

So I dug a little deeper and realized I had two versions of the ZapfHumanist font installed: One a True Type font from Corel (like Lagarto suggested) and one from a Bitstream font collection which identified as an Open Type font.
I removed the Bitstream Open Type font, reloaded Publisher, and all is working as it should now.
Pauls, I can upload the font, publisher doc and a PDF if you still require it. Just let me know.
Thanks for the help, but sorry for wasting everyone's time.
With thanks, Lee