To be honest; Don't listen too much to random people on the Apple Support pages. While there are also people on here that might lead you down the wrong path, Apple's Community Support Pages (i.e. the ones not made by Apple themselves) are generally less useful than these forums in my opinion, and some of the advice in the post you made there is definitely just leading you down wrong paths, like worrying about memory and disk speed. While these can be limitations to networking speed, a 5400RPM hard drive would not limit your networking speed unless your network is at least 560Mbps; And that's with a pretty full and fragmented drive. And unless you're very memory starved due to running a billion apps, your RAM shouldn't be causing this either. I also doubt you have malware, but if you got the Mac from someone else you probably should've erased it entirely when you got it, but let's leave all that aside because you've already said the network speed was higher a few days ago.
Yes, you should run csrutil enable in Recovery mode. I would advice just getting an adblocker extension or finding another solution than a VPN. With a VPN all your network traffic goes through a server the could be halfway across the world before going to its destination. A server that could be keeping logs on everything you do as well. VPNs have their place, but I'd personally never connect to any VPN that isn't my university network, something I have control over, future employer network for a very specific temporary purpose like CTF challenges. It could very well be slowing down your networking to be connected to a VPN since in addition to your own connection and the additional latency of connecting to another server, that server's speed is also a factor and it could be handling other computers as well.
Other ad-blocking solutions that live either as a browser extension or on your network can stop the connection to adservers from being attempted which will improve browsing speed on heavily ad-filled sites, in contrast to your VPN that will even in optimal circumstances, inevitably, be a slowdown, even if it isn't the cause of your current extreme slowdown.
But I would also advice allowing ads on pages you enjoy, since it helps the sites stay open for business. Either that, or when possible send them a buck or two to offset the lack of ad revenue.
Should you desire an adblocker they're available both from the App Store and various places on the web. uBlock Origin, NoTrack, many options
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