Apache has a bad reputation: for being slow, for being bloated, and most importantly, for suddenly freezing or slowing to a crawl just when you need it most. Like, say, when you've just found a campaign with massive scaling potential.
I've heard a few people mention that they've had campaigns which seemed to be server-limited - whenever they increased the traffic beyond a certain amount, the campaign would just die. And it's quite likely that your server's configuration is the problem there.
But here's the good news - for Apache, in particular, there's one setting which is responsible for the vast majority of server crashes. It's very sensible in design, it looks very innocent - and if you go over a certain level of traffic, it'll kill your server stone-dead, with no warning.
Enter the villain: Keep-Alive.
How Keep-Alive Makes Your Server Dead
The Apache Keep-Alive setting stops your server from instantly dropping connections the moment they're finished. That's actually very sensible: otherwise, every time one of your visitors requests a file (and on an average landing page, they'll probably do that about 6 times), Apache has to start a new connection, which stresses your server's CPU out.
However, if your server gets hit by a mighty stream of traffic - like, say, you increasing the spend on your Facebook campaign - Apache's default KeepAlive of 5 seconds will proceed to quietly and efficiently murder your page load speeds.
Apache runs a fairly small number of connections at any one time - probably around 20 on a low-powered VPS. Assuming your landing page loads in .5 of a second, not counting DNS lookups and rendering time, Apache will send all that information out to the first person who clicks on your landing page - and then it'll keep that client alive for another 4 and a half seconds, doing precisely bugger-all. Meanwhile, more people are coming in - and once 20 of them have come in within a time of less than 5 seconds, all of Apache's connections are used up, and the next person to ask for a page gets put in a queue to be served once a connection is available.
If you've got, say, 10 people arriving a second, it's easy to see how this can go badly wrong. For the first two seconds, all's well - but in the third second, there are no connections available, and so the 10 people who clicked through have to wait 4 seconds for Apache to have free connections for them. By the fifth second, the wait's up to 7 seconds. And it just gets worse from there.
How To Fix The Problem
The simplest fix for this - and the one I'd recommend for most Apache setups - is to turn KeepAlive off.
This is very easy to do.