I have mid computers from 2010 running that take far less than 2 seconds to open a browser, I think there’s either some missing driver for your hardware or something wrong with your hardware in the first place. Please post your exact specs so that we can try giving you better advice.
Also worth noting that for modern-ish computers the desktop environment is the least offender when it comes to resource consumption. Any modern browser will use roughly at least 2x memory compared to the desktop environment.
The CPU is alright and the RAM should be sufficient for most tasks.
There are a couple of things that could impact performance here, first the SSD could be on its way out, or slow to begin with, so maybe post some info on that.
You can use gnome disks to get more information on your ssd, here’s a screenshot on my system, with some information obscured. The model name should be enough.
You can benchmark the SSD using either gnome disks again, as shown here:
Or alternatively you can use KDiskMark. Give us some numbers, maybe it’s gonna be as simple as replacing your SSD.
Another thing that could make your system feel sluggish, since you lament little freezes happening, is Plasma itself. I know what I said about desktop environment havyness, and I still stand by it: it’s not that Plasma is “heavier” or anything like that, it’s just that some of its code is not very well optimized and with slower drives it can show. Brodie Robertson made a video about this a couple of years ago, see if what he describes matches your problem, but consider that this is old information at this point and while some of these issues might still exist, they might have already been fixed.