This is currently happening on my end as well. An Apple Advisor said Apple has received reports of the same and are "investigating" whatever that means. I sent a report via Feedback Assistant and so we shall see if this gets fixed. It's not only making Contacts slow, but also Messages and Mail which pull from the contacts database. Strange, indeed! I have even removed all of my contacts from iCloud on my computer and re-synced the database. It didn't work. Back to being as slow as molasses!