a few things you can check:
- try running
netstat 1
in the console and checking the statistics. if your gamemiss value is high, you're suffering packet loss, which could be caused by all kinds of things, some of them you can fix yourself.
- when looking through the server list, check the server that has the lowest ping (should be sorted by ping by default). if there are servers with <100 ms ping, then chances are you've just been joining servers that you don't have a good connection to (which i suspect is the case since according to the speed test, you live in Melbourne, which is pretty far away from Europe and USA where most servers are hosted).
- i've seen many people complain about lag when in reality, it's the game being laggy. run
showfps 1
in the console and join a server, and if your fps counter drops, it's your PC that's slow. if so, my only advice to you is to lower graphical options or if that doesn't work (which i've found is almost always the case because SRB2's biggest bottleneck is CPU), you might need to upgrade your PC hardware.
but as with all of these issues with ping, it's really hard to know the cause since there's so much that it could be wrong with. the internet speed test is good information, but there's so many more factors other than your connection to
specifically CloudFlare (since that's what you tested) that's the problem here, since what you connect to can impact latency and packet loss on your ISP due to different PPP routes.