Autologin feature killing my website I have a drupal 7 site.

Running a auto login feature when people click on my newsletter.

A hook is fired when they visit the site to check for the cookie.
my setup : 
Background info : I am already using Memcache, APC, Varnish, 

2 Varnish Servers (8 GB)
1 Db percona server (32 GB)
1 HaProxy 4
Webservers, 16 GB

my question is, Performance wise. I can see that my server cannot handle 8000 users logging in. 

Is there any way to make this impact on the server more "soft" in terms of performance. So my DB doesnt go down. when 8000 request auto login at the same time.

Example 1. 

Ex: 1 request to auto login, causing 3-4 query to DB via  the core Drupal API if so if I get 1000 users at the same time I got 3000 queries to DB -> DB cannot handle the load. What to do ?!

Best regards

Victor
You could cache the content and displaying it cached would ideally take stress off your database. Check out module "Boost" it can help.

Ideally optimization of the site is what is needed along with better hardware and maybe Varnish.

If you are using views consider using views built in cache with time based expiration to take stress off database. 

Update

If you already have Varnish, APC and memcache then the only other think you can really do is get some better hardware. 

We have servers that get about double what you have and they run on a quad core processor with 32GB of RAM. 

We also split Web and DB server so we can utilize APC for Web Server and memcache for DB server. 

Have you checked your APC stats? Varnish probably won't do much unless you configure it to cache Authenticated traffic which by default bypasses Varnish. 

What are your server stats?