A range clear is typically a very quick operation to perform from a user perspective. In the ssd storage engine, it basically just detaches the affected data from the b-tree and defers the cleanup work. In the memory storage engine, it removes the data from the memory data structure and logs the mutation to disk with no deferred work.
The cleanup work in the ssd storage engine is actually a fairly slow process, and it consists of two phases. The first phase processes the detached pages and makes them available for reuse within the file. The second phase removes them from the file. As long as the first phase keeps up with your write load, then there shouldn’t be an issue. If it doesn’t, increasing the size of your cluster should help. See Used disk space dramatically increases while sum of key-value sizes is constant for some additional discussion on this.