> 300 km/h trains connecting Finnland with Spain with no delay or bumps?

Bit tricky this: either you cross the Baltic by ferry and resume at Tallinn, or you have to go a long way round north from Helsinki and come down again through Sweden, across Oresund and through Denmark.

> 300 km/h trains connecting Finnland with Spain with no delay or bumps?

Have you seen a map of the region?

The land border between Sweden and Finland is in the far north where few people live. It would be preferred to not do switches there because that means you need to build/maintain a town for all the needed workers (and families), and said town won't provide much opportunities for any other jobs. In short if you must switch gauges you really want to to be in a smaller city (maybe 100k people) that has other reason to exist so you have a pool of people to hire, who have other reasons to live there and thus have family there.

I hadn't considered throughput but that's a good point. Under normal circumstances even a few hours is meaningless but if you need to get lots of trains across the border in a short span of time it starts to accumulate.