Sorry to be the bearer for bad news, but your base Macbook Air 2010 is slow by today's standards. First of all, it is a Core 2 Duo so it is not a speed machine unlike your Mac Pro, Mac Mini 2018 and iMac which can easily smoke it. And if you are coming from these fast machines, the Core 2 Duo will seemed like it's lagging because your visual perception is coming off a faster machine. Also, it comes with 2Gb or 4Gb of ram, which today is simply not adequate for modern browsing. You need at least 8Gb or 16Gb with Chrome. Also, the video ram has a paltry 256Mb (enough 10 years ago), but sorry to say not at all adequate for modern day browsing where you need at least 512Mb or better 1.5Gb or 2Gb of VRAM to speed up the screen rendering. Any modern day browsers utilize the GPU and VRAM to render and store web pages in memory. Your Macbook 2010 Air just doesn't have that, so what you are seeing is that it madly pages between your SSD and memory/VRAM and that's why you see the lag. I would not advise you to upgrade the SSD, which will be a total waste of money. Use it as it is or else install Linux like Ubuntu which will use far less resources than El-Capitan does, PLUS has security updates on some versions up until 2023. Whereas El-Capitan stopped updating last year. Besides, you still get FireFox, Chromium and Opera (if you need VPN support) in Ubuntu and it flies. My older Macbook 2007 which is running Ubuntu 14 LTS is being used by my dad. Very speedy and little lag and it does not even have an SSD. I work in a computer thrift store and recycling depot and we routinely get a lot of these 2010 Macbook Airs and while we loaded them up with High Sierra (the max they would go), they are way slower than my Macbook 2007 in terms of browsing speed. That tells you that Mac OSX can be bloated and is slow on older hardware. I myself installed Linux Mint 19.2 LTS on my Mac Mini 2011 and configured it as dual boot. In El-Capitan, my Mini browsing speed is fast, but under Mint 19.2 it is super fast due to its lower resources and so I use Mint for browsing, but El-Capitan to run Mac apps.
