The
top command shows you have 5 instances of Chromium running. That shouldn't be a big deal, I don't think, but you might try running just a single instance since slowness is an issue.
A full re-install would have to be done from a USB or DVD, and you'd tell it to use your whole hard drive... so everything that is on it would be erased. I like to do that because it makes everything new again, but that's drastic and not everyone agrees with my philosophy about that!
But it also a good time to experiment and try a different distro. Since you're using Ubuntu now, you might could try Linux Mint, or Linux Lite, or something else to expand your horizons a bit.
I don't know that I'd say Opera is "better"... I have used it for many years and I'm fond of it. But I like Chrome/Chromium and Firefox too... I use all of them. On my Android phone I also use the Dolphin browser.
Trying to solve a "slowness" issue can be difficult. Sometimes it is just a perception... you think it is slow, but it could be that it is working fine for your computer hardware, your RAM, your particular desktop (Ubuntu Unity is not the fastest, but it should be adequate at least). Like you mentioned before, it may be that the "slowness" is the Internet, and not the computer at all. You said the computer starts off okay... so maybe just more frequent reboots is a simple solution (because, for example, a true cause could be a bad RAM chip, and we will likely never discover that). If you have a hardware problem, like a bad RAM chip, then re-installing Linux or installing a new distro will not ultimately help that, but it might lead you to realize there is a hardware issue. Trying to sort through all the possibilities can be fun, or frustrating, depending on how you look at it.