Uhm, well, from what I've noticed the GOG website is horrendously laborful for all web browsers in general. Pretty much each and every page is just huge, and several upon several files, and tons of JavaScript and tons of CSS, and from several different sources on the Internet. The GOG website is not easy to render, and not even possible if certain files fail to load because they might be needed to request further data not included in the webpage at first. So, if certain key files take a little longer (perhaps because other files in the queue are taking longer…) then that can delay matters, too.
I'm also thinking that to you it seems difficult for the web browser (on the OS that you are using) might also be a matter of how the processing is revealed. I've noticed lately (with most recent versions of web browsers) that sometimes web browsers will wait to render any more of the page, not until the rest comes into view. For example, would you say the scrolling is still sluggish after scrolling all the way down and back up again? Or does scrolling become okay after the whole page has been viewed? I don't mean when visiting the page again, I mean while staying on the page and casually scrolling after having scrolled the whole page. Also, are you scrubbing quickly or just casually so as to give it time to figure everything out the first time through. It might decide to skip further processing because that part of the page didn't stick around long enough to get processed, so it might still need to work on it the next time it comes into view and stays long enough to be "worthwhile" for rendering.
In other words, it could be you're experiencing a feature, perhaps "Just-In-Time" rendering rather than processing everything that might not ever be viewed. Marketing has been saying such a feature is more energy efficient, or that it gives the appearance of finishing quicker by not downloading images until needed, or only computing image size and placement without immediately generating its view. That might be more noticeable with certain combinations of OS + web browser + web page (of some website). (Assuming the Internet doesn't hiccup too much differently during testing of each combination.)
I don't believe same brand of web browser matters when on a different OS, just like same brand of OS doesn't matter when using a different web browser. I mean, I think that there happens to be a different experience with a different combination is unsurprising, and so I personally wouldn't label it as a problem or issue. Noteworthy, sure. I'd want to pick the experience that's more comfortable if I were to have such different experiences, or at least play with it enough to figure out what approach helps make a more comfortable experience for the different circumstance. Not every syrup dispenser works as smoothly with the same approach, even in the same restaurant.
