If the registration process just picked a random instance for you, maybe something nearby, and assured new users that they can visit communities and interact with users across instances, very few would pick the biggest instance.

First and foremost, the user that adviced you to look at the connection speed is 100% right. If you establish a gigabit link with a gigabit device, it makes no sense to upgrade other than future proofing.

There’s no point in going beyond Cat 6a. Keep in mind that the length of the cable is a big factor as well. For 1/2.5GbE, Cat 5e is plenty (for at least 100m). If you plan on going to 5/10GbE now or in the near future, Cat 6a will get you there with ease (for at least 100m). In both cases, keep the cable as close to the required length as possible + a 30cm/1ft service loop (slack) on each end. That will cause no more signal degradation then necessary and make for a clean install.