[Sabardeyn]

Kovid is, of course, absolutely correct. I did want to add something for yourself and other users to consider.

If calibre is running on an older computer system, with limited memory or a less capable cpu chip, it will run slowly, in just the manner you have described. However, calibre's ability to process actions and perform tasks/conversions does not generally change. Over the course of several updates calibre should maintain the same relative speed. It should not noticeably slow down.

There are some exceptions: poorly crafted ebooks with too complex of an internal structure; or a copy of an ebook that has been converted too many times (conversion of a conversion of a conversion situation). Other possibilities exist too. For instance, if you download metadata and have thousands of tags, or series names - many of which are spelling or alternate wording of other entries. Or just a humongous library with tens of thousands of books.

All of these things can cause calibre to seem to respond slowly, but are usually temporary delays until the specific task is done. You can improve calibre's response time somewhat by cleaning up your metadata.

It also helps to keep the Tag Browser and Book Panel (left and right sides of the default view, respectively) closed until needed. Use them as needed and then re-close them.