Not everyone has the same weaknesses, I believe the Problems are based on the ratio/amount of participants that complete the problem, if you struggling on a problem you consider low rated, it probably means your weak in the a subject, and should target it when you practice next time

There are some harder (2000+) problems that I can solve fairly quickly, and there are some easier problems I still can't solve (there's a 1000 rated problem that I've probably spent over an hour on and I still can't solve it). The rating is based off of how most people solved it, so your personal results might deviate from the actual rating (and sometimes, there are easy problems that you simply can't solve, like I mentioned in my first sentence).

theres a very complicated algorithm. It can be reduced to something like this

int getRating(int problemID){
    return ((rand()%28)+8)*100;
}

hope that helps!

The theoretical rating (the rating of a participant that has a 50% chance of solving) is calculatable from the rankings. For example, https://clist.by calculates them. The rating on CF is usually quite close to this rating, but maybe a bit lower (sometimes the [theoretical] rating can be inflated).