I've used it for years and love its features but yeah the UI and processing always felt heavy like a java app but as someone else noted it's not java - it's python so I suspect the laggy of slowness in the UI might be a side effect of python's single threaded / GIL performance issues. Python can be fast, especially for scientific work but in my experience - I get annoyed by python apps more than other interpreted languages such as Ruby as it often seems to peg a single core to 100% whenever I'm waiting for something to finish processing - again I'm sure it's the way things are written more than the language itself as such but it seems it makes it hard to write python applications that make efficient use of modern multi core CPUs. (I host a lot of Python, Ruby and web apps written in various other languages).