Fast hash function for set equality
MinHash already is super-fast.  I suggest you check whether you understand correctly how MinHash works.

Take any standard hash function $H$; then all you have to do is compute $H$ once on each element of the set.  You can implement $H$ with as fast a hash algorithm as you want.  The update operation requires one call to $H$ (namely, if your set's hash is currently $y$, and you add element $x$, the new hash is $\min(y, H(x))$), and the union operation is super fast (namely, the union of two sets with hashes $y_1,y_2$ is $\min(y_1,y_2)$).  Notice that these operation have the property you want (adding an element already in the set does not change the hash).

I have a hard time imagining how you could get much faster than that.

If you want this to be faster, a reasonable way is to choose a $H$ that is faster.

I've described a variant where the hash of a set is the smallest of the element-hashes.  The only downside is that the number of hash collisions might higher than you like.  Measure and see if that is actually a problem in practice.  If it is, you can consider variants that keep the $k$ distinct smallest element hashes.  If $k$ is large, you can store those $k$ hashes in a heap of size $k$ to make the update and union operations faster.