It's not clear that two or more signature verifications are more than decrypting one message.
It's not clear that any conceivable number of transactions received would constitute a scaling problem.
Note many people running block generation now verify all the transaction input signatures on all received blocks and transactions. They still have plenty of time left to mine for more blocks at rates of millions of tries per second. The whole process continues even when the net is tossing transactions at them as fast as they can generate them. I don't think we know for sure where the first scaling bottlenecks will be but my bet is it won't be CPU time.