Not possible as explained by Jackie. To get that low respawn timer, you would have to be playing in a server of less than 8 survivors? Like I mention before, it's most likely the infected is on a good rotation respawn timer.
Infected 1: 15 second to respawn
Infected 2: 7 second to respawn
Infected 3: 3 second to respawn
Now that's a good rotation of infected respawn timer, each one of them spitting and delaying the advance/incapped another 5 seconds. Once infected player dies, the next infected player spawns and goes in and repeats. Typically we see the infected doesn't work together and they all die around the same time, thus their respawn timer is very close to each other. Most noticeably during the beginning of the saferoom exit.
Currently emtpy on ideas right now...we should just roll along with what we have so far.