Is tai chi just form?

Form is just 1 area of practice in tai chi.

8 areas of knowledge and ability provide a balanced comprehension of the art:

1. Qigong (energy/breath work)
2. Form
3. Pushing hands
4. Meditation
5. Theory & philosophy
6. Martial skill (kung fu)
7. Neigong (whole-body strength)
8. Instructing (optional)

A tai chi student should work towards competence in each of these areas and also uncover the essence of the art.

Isn't tai chi slow and boring?

Some tai chi exercises are slow, whilst others are not.
Tai chi chuan (martial art) training starts slowly and then speeds up as the student becomes more skilled.

A good class will always have a comprehensive syllabus in place; ensuring on-going development and access to new material.
The training challenges your mind with unexpected insights, skills and possibilities.
No two lessons are the same.