Check your favourites. When I import all my favourites from Chrome & active iCloud Safari sync, Safari works sooo slow and shows beach ball. It can't handle a lot of folders and links. Open Safari without any tab should be fast in nMP 32 GB ram, but sometimes it opens after 1 minute!!!
In Chrome everything works fine, it can continue work and load more than 100 tabs when Safari can't load one window without any tab! So the best way to increase speed - delete all Safari bookmarks and uncheck iCloud synchronisation.
Clean Safari load in ~1 sec.
You can find your Bookmarks file in ~/Library/Safari/Bookmarks.plist
Close Safari, try to rename this file, open again and check if it helps. If it works as previously, you can always rename to Bookmarks.plist and it will load all your bookmarks.
Don't forget to remove history items.