> With virtual dom library I'll just update my data and rerender everything that depends on this data

So you have to visit every data node, and then also visit every changed UI node. Where if you have deltas, you only visit changed data nodes and then changed UI nodes.

Re: data snapshots, it's easy to design your own service to use a delta protocol. But even when you can't, you can separate the code used to construct your reactive objects from the code that initializes them. This is just basic function abstraction, and it doesn't really add any work. From the example on S.js site:

    const                                // 
        a = S.data(1),                   //     a() |   1     3     3     5
        b = S.data(2),                   //     b() |   2     2     4     6
        c = S(() => a() + b()),          //     c() |   3     5     7    11
        d = S(() => c() * a()); // t0    //     d() |   3    15    21    55
    a(3);                       // t1    //         +------------------------>
    b(4);                       // t2    //            t0    t1    t2    t3
    S.freeze(() => {                     //    
        a(5);                            //    
        b(6);                            //    
    });                         // t3    //

Now becomes (quick and dirty to convey the idea):

    const
        a = S.data(1),
        b = S.data(2),
        c = S(() => a() + b()),
        d = S(() => c() * a());
        update(3, 4);
        S.freeze(() => update(5,6));

        function update(aval, bval) {
            a(aval);
            b(bval);
        }

S.js will detect whether the value you're providing is actually different, and will only propagate what has changed. Roughly the same number of lines of code, just more reusable.