Sorry for the late show. Had one site I couldn't access after disabling all plugins. That is fixed now.
Back to the slow /wp-admin/ access. That is still there. My server is a 4Gb RAM VPS in NL. I don't think they throttle me, and anyway, the pages load fast. I have some rules for the: /wp-admin/ access.
Not sure about the bots. I don't see malware (I know how that looks, had that before, not on this server), but there are certainly many failed login attempts. I can login only from a few IPs. A reach to admin from the wrong IP will trigger an instant block.
When I look at the "bad" traffic on Wordfence I don't see lots of hits, usually below 100 per hour. That should not really effect the performance I believe.
Where can I look at next? Maybe some site settings are not so good. My server experience is rather low.