It's true that FTM scales very poorly. The more of anything it has to deal with, the slower it is. It shouldn't, however, have much trouble with the numbers you've mentioned unless you have multiple media items per person.

I would merge these tree files, however, if you have a lot of overlap. There's no sense in duplicating information and media. That only invites errors and omissions and wastes your time. I personally would only leave them separate (from an research/organizational standpoint) if I had a compelling reason to do so.

(Edit: Added PS below)

It's generally bad form to include surnames in a posting in a forum for genealogy software. That makes this discussion thread show up in searches people do for those surnames, and this discussion isn't related to researching those names. It only adds to the clutter someone must wade through with an interest in those names. As a Norwegian researcher, I'm sure this is something you can appreciate. There are a limited number of patronyms that we're all researching, and farm names, although more unique, tend to be repeated as well in their many varieties.

We benefit as a community when we restrict ourselves to using the "surnames" field only for postings directly related to those surnames. My advice to you is that you edit your original posting and delete those surnames. If your goal is to connect with others researching similar names and farms, then post something specific in an appropriate forum.