Same issue on my site - this has been ongoing from about the last 7 days. One way I was able (I think) to partially resolve is by blocking all foreign IP addresses (we do all of our business with North American companies so blocking foreign traffic is not a concern for us at all).
This is just a very constant stream of attempted logins and so far it's only impacting one of our sites.
Performance has not yet been impacted and I can login and modify the site as needed. I am having wordfence auto ban any IP that attempts to login with a non-existent name. The issue is that every login is using a unique IP so this is not a super effective means to stop the problem.
The only semi-answer I've found is blocking foreign IPs and so far that has done nothing to reduce the amount of North American attemps.