Does inefficient code cripple the performance of an app?

It depends on where the code is. If the code gets called rarely, it is unlikely to cripple an application. If you are calling the code constantly, and it is wasting cycles, it is possible that you may deliver a poor user experience.

I can offer an example from many years ago, when it was easy to measure performance. We had an application that was so badly-written that it gave continuous errors, so I asked a member of the team to rewrite it (if you think it might need re-writing, believe it - it does). The original (batch) program took over four hours to run, and the replacement took 30 minutes - and eliminated the errors from the first program. To an extent, that's the difference between a good programmer and a bad one. The second version wasn't particularly optimised but it was written by an experienced programmer who organised his logic properly.

Innefficient code is likely to slow an application down, but in truth most applications spend 99% of their time waiting for user input or waiting for data to arrive from the database so the inneficiency of a particular piece of code may never actually be noticed. So it really depends on the number of users/amount of traffic your app will have to cater for. I kind of agree with Rob - people can worry too much about performance too early on, although my counter argument would be that 80% of performance is in the orginal design of the application... so a well designed application with bits of inefficient code is likely to be easier to fix than one that is badly designed. There are some obvious areas where code efficiency is crucial; in embedded systems, for example, developers often measure precisely how many CPU cycles a given function will require, and then agonise about finding smart ways to reduce the number.