My mother's iPhone 11 or 12 (forgot which one of the two) works perfectly fine. I think performance in general has a complicating factor, because iPhones are downclocked when the battery has degraded to avoid voltage spikes that can lead to instability with a bad battery. In many cases, performance improves after battery replacement.
iPhone 4 and 5 are really a different era when smartphone software and hardware was still developing very rapidly. I mean, the iPhone 5s alone introduced: a 64-bit CPU, a secure enclave, Touch ID, the first iPhone with separate co-processor to process motion data. Similarly, the iPhone 5 doubled the RAM compared to the 4s and had roughly twice better CPU and GPU performance than the 4s. Such changes are unheard of nowadays.
iPhone 4 and 5 are really a different era when smartphone software and hardware was still developing very rapidly. I mean, the iPhone 5s alone introduced: a 64-bit CPU, a secure enclave, Touch ID, the first iPhone with separate co-processor to process motion data. Similarly, the iPhone 5 doubled the RAM compared to the 4s and had roughly twice better CPU and GPU performance than the 4s. Such changes are unheard of nowadays.