I solved the slowdown by first gluing everything together to form 1 larger string:
Identifier string (just 2 capital letters so Labview knows what the value means)
Value string (the actual measures value)
and a "\n" (I'm using this in Labview as a separator)
Sending it as one big packet reduced the time from 17 ms to 1,5 ms which is acceptable to me. Since I'm doing active checking if a set interval has passed 17 ms would mean I could be of by up to 17 ms. (This is the scenario where it arrives at the checkpoint just before the interval passed and would then only execute a full iteration later). Sending it only every n iterations also wouldn't cut it because I would still have the same problem except this time the problem would only have a chance of occurring every n-th iteration.
With this 1,5 ms sending time I can actually remove any risk since I can simply execute it just after a new interval has been set (all my intervals last longer than 1,5 ms). And in the future when I need faster SerialUSB I will just get a Teensy.