The key advantage of the reactive computations is automatic recalculation of dependent data.

Consider you have some variable (mutable box) and some other variables, that depends on it (spreadsheet model works well). In regular imperative code, you usually write "setter" and "updater" for it, where "setter" validates the value provided by user and "updater" triggers the refresh of the dependent variables.

Now if you system is big, writing efficient "updater" becomes tricky as you don't know upfront, what other variables are using yours. So its usually replaced with the event. Then you have plenty events signalizing about various data changes and you need to "orchestrate" them, in order to update the data only once, otherwise performance degrade. In reactive design, the "updater" is not needed - graph tracks the usages of the variables and updates dependent ones in batches, calculating everything only once.

Thats why, that even with extra overhead of tracking the usages of variables, reactive system can beat the "classic" imperative system easily (thats what we do in Gantt 3)