Wordpress heavy load / alternatives to it

I am making a website for a client who is basically creating a website which is an archive of information of a certain topic. The amount of "posts" he has is near 500k or so, quite a lot for wordpress. I've tried inserting them all with a script to see if it could handle it and I've found that it was able to handle them.. at least on the frontend. The backend, the admin area, is basically unusable right now. It will crash after any change you attempt to make.

Now, this was done by using just w3 total cache, yoast seo, akismet, and the default theme (which is kinda nice with the pagination job).

My question is, are there any known solutions for heavy data on wordpress without having to modify its core? I've seen a few lightweight alternative to wp out there, but wp features/plugins are a nice thing to have.

Thanks.

via Chebli Mohamed