Hi Patrick,
I know this answer may come late, but I thought I would reply anyway so there is something here.
Generally speaking, horizontal scaling (i.e. multiple instances, load balancing) will help where you have a large number of requests to your app, as well as increase availability (there are other things to consider with availability).
Vertical scaling increases the server resource (memory, processor, storage) per instance and will help where you have resource intensive processes, as these will only run on a single instance at a time.
So it really depends on why your app is under performing. There are other things you could also do to improve performance, such as optimisation, queueing, microservices etc. as well.
I hope this helps.
Hello,
Refer this link https://www.mendix.com/evaluation-guide/app-lifecycle/mendix-cloud-overview/#6-how-does-horizontal-vertical-scaling-work-in-the-mendix-cloud
Hope it helps!