That's certainly high enough to affect the feeling of responsiveness. It won't feel as "snappy" as it would without the latency. However, it's easy for a difference in server-side processing speed to dominate that degree of latency. Lemmy isn't super optimized and different instances can run on wildly different hardware with different levels of care put on DB tuning.

I kind of noticed that it's a bit slower if the ping is high (another continent)

On which continent are you located?

Some of the apps will struggle with higher ping rates. I have some issues on thunder

For web browsing? No.

Not really that important, the first load might be really slow, but after that it should be smooth. The api responses will be slower, but it doesn't matter that much.

Not really.

I joined early stuff was unstable. I couldn't join one that was in my country (UK) and eventually managed to join one in Oz without knowing - just my luck!

I don't have any noticeable delays in content loading unless there's a general problem with the version of Lemmy.