I found Seed, which is basically a JavaScript environment and binding for programming GTK+. Are there any limitations compared to using e.g. C and C++? Are there any major speed concerns?
Sure, it's potentially slower than writing in C or C++. But it also kind of depends on what you're doing with it. It really depends on how much complex logic is in the Javascript side of the app.
If you're writing a lot of custom widgets that draw themselves from Javascript, it's definitely going to be slower.
If you're writing in Javascript but you're using all widgets that are part of GTK+ (buttons, treeviews, etc) then you probably won't see any slow-down from the user interface. If you have insanely complex logic going on during your callbacks and stuff then you still might.
There won't be any "major speed concerns", and in general you shouldn't find it to be much slower than C or C++ if you are writing good JS code.
Most of your overhead will be in the database / filesystem on a typical application. If you plan on doing complex computations, you may see a difference.
In short: it depends on what you are doing with it, but in general I wouldn't say that you will see major speed concerns. It has worked well for me.
If you think it could be done in Python, JS will work just fine.