It is not a bug.
Creating and manipulating ebooks involves file compression. That is very, very CPU intensive. Also calibre takes a conservative approach to updating the library and the books, and work on copies in the temp directory. This means a lot of file copying and transfer between different folders and drives. Also takes power and may cause a need for extra cooling. But it also means that the library and books very rarely become corrupt.
Some software may throttle the use of the CPU, to avoid hogging all the resources. Calibre has some settings in the preferences that allows you to lower the number of cores used for some tasks. That may help in some cases, but may make some jobs take longer to complete.
Install more memory if possible and make sure you have plenty of free space left on the hard drive. You might even consider upgrading to a faster hard drive.
Other than that, try cleaning the fans, and make sure they are not obstructed!
I have a 8 core computer with 16 gB RAM and SSD disk and temp folder in RAM, and calibre makes good use of it all!
Lycka till!