Runtime cost of using Clojurescipt is undeniably there but for most applications is pretty negligible price to pay for the big wins. In practice, Clojurescript apps can often perform faster than similar apps built traditionally - especially for things like render optimization - immutable data enables cheap equality checks for memoization, it prevents unnecessary re-renders; data transform pipelines - transducers give you lazy evaluation, it's great for filtering/mapping through large datasets; caching - immutable data is safe to cache indefinitely, you don't have to worry about stale data;
You guys keep worrying about some theoretical "costs" - in practice, I have yet to encounter a problem that genuinely makes it so impossibly slow that Clojuresript just outright can't be used. Situations where it incurs a practical cost to pay are outliers, not a general rule.