Yes, watching paint dry / watching grass grow means that an activity is very, very, boring. This is certainly understood by all native English speakers, because it is used once a while (but not every single week). English is not nearly as colorful or vivid of a language as Chinese, when it comes to idioms or how much information you can compress into few words (Chinese can compress a ton of information into just 3-4 words but English cannot do that). In any case, other English words that describe really boring are: "tedious", "mind-numbing", or even better, using these two together: "mind-numbingly tedious"

I would say it is not used particularly frequently. You could also say it is dull or uninteresting, or unexciting.

It means "very boring." I'd say it's medium-common. Native speakers definitely understand it.

Yes, it means boring. I'm Australian and both are fairly common phrases here.

I've never heard the spider example but variations on "like watching paint dry" are very common. I've also heard the grass one before but not very often. You're right about the meaning.