As you have already read in this guide, the general performance of the new 1.14 versions is much worse than in previous versions.
According to some reports and our own tests, the performance of 1.14 improves a lot if they have set a reduced render distance.
For this reason, all Vanilla 1.14+ servers have a reduced default render distance. You can change the render distance in your server options.
For better performance, we recommend switching to the PaperMC server software.
The reduced render distance is not the cause of why the mob spawning is broken on 1.14, more information about that here.