Login screen loading slow

My login screen is extremely slow to come up. That is, if I go to bloglocation/wp-admin it takes forever to redirect to wp-login.php.

I have multiple WordPress network installations and this is affecting only some of them. All are them are running the newest WordPress version and have the same plugins.

I tried reinstalling one of them. Until I imported the old data into the database, the login page was fine. Afterwards it was slow again. Are there any values in the database that may be responsible?

I’ve tried disabling all plugins as well and that makes no difference.

Thanks!