Is it efficient and a normal practice to have one class with thousands of code?

I am currently in a continuous software product development project on base of legacy Java code. The source code is very complicated, which is good and bad.

But I am surprised to see that in the core component most of the classes contain thousands of lines, not one thousand of lines but usually over 5000 lines, in only one .java file. As a result, developers, or at least me, often times spend pretty much time on using the debugger with IDE. IMO, such way of coding lost the purpose of object-oriented programming and definitely need refactoring

I don't know the cause of spending much time on debugging is because I am a jerk without enough experience or the thousands of lines of code in one .java file, and I sometimes really got fed up with such kinda code. I once decoupled some parts of code in separate java file but some team members criticized on this with lacking of teamwork spirit as an excuse.

Question:

• Is it worth and valuable to work in such kind of software development project?
• Is the lack of time an excuse for adding hundreds of methods to one Java class and hundreds of lines to only one method?

I faithfully would like to get comments and opinions on this from some more experienced developers.