If the data is < ram size and if you read that data again and its off disk again its the slowest it can possibly be, there's a reason most databases implement a buffer cache (actually making writes insanely faster as well) but yeah, MySQL is generally not a very good operational database with all the ones I have tinkered with.

You paid one hundred bucks for that eight gb of ram, do you really want it to just sit there unused?

No, but my manager was wondering why our website was slowing to a crawl.

Is the DB on the same host as the web server?

It is more likely they did not leave enough overhead for the host operating system, which is a classic issue.

I don't really remember, to be fair this was nearly 10 years ago now. Upon some googling now, I do see a way to limit just how much Mongo sucks up for data + index. I am curious if it would have been a smoother experience, if this configuration was even available then.