I have Googled this question at various points over the years, and I must say I am annoyed at how many "experts" don't even address the question but instead use it as an opportunity to show off what they think they know.
I can only tell you my own experience, without any nerdy jargon. I have found that YES, having too many bookmarks does slow down your browsing experience quite noticeably. I use Firefox and Chrome, and I clean my registry at least one a week with Glary Utilities.
I am also a bit of a bookmarks hoarder. Often, if a page is interesting to me, I save it as a bookmark, probably adding dozens per week, sometimes just because I'm lazy and want to come back to that page.
Once I've got about 100 bookmarks, I notice that my browser starts feeling very clunky. So, I go to my bookmarks, save them in HTML form to a folder on my computer then delete almost all of them from the browser, leaving just the main ones I visit, about 10-15 bookmarks.
And BINGO, my browser is fast and clean again. I end up doing this about every six months.
So, short answer: YES.