I used to only have a problem under windows, but now I seem to have problems on windows, raspberry pi and linux. It's slow to start - (takes forever when checking boards), and now the board manager takes forever to open, library manager takes forever to open and updating any items under either takes forever to finish. I've tried uninstalling-reinstalling java, I've dried using the jdk versus the jde. I've tried disabling virus checking. I've tried removing my Arduino and .arduino15 directories. What the hell is slowing this thing down so much? Trying to research it I see posts all over the place about how unbelievably slow it is and nothing concrete identifying a specific problem no less a solution! Just lots of people dumping huge lists of start-up debugging info and a whole lot of 'ditto/bump' posts with no real answers.
There are lots of "concrete" answers but it often just takes time to read through all the posts and not just the first couple of replies to a Q.
Under windows it is often related to security software, lack of user privileges, using network drives, other sync software for other devices, messing with folders / install directories, and a long list of other probable causes.
Best to symptomatically address each item and maybe also run the DEBUG version to see if it throws any light on the topic.
Can speak for the Pi though...sorry