You cloned every word, which is much worse than cloning the vector (which only clones a couple of pointers). Here's the performance of the original code: ===>

You cloned every word, which is much worse than cloning the vector (which only clones a couple of pointers). Here's the performance of the original code: ===> ./phone_encoder Proc,Memory(bytes),Time(ms) ./phone_encoder,24584192,198 Proc,Memory(bytes),Time(ms) ./phone_encoder,24584192,461 Proc,Memory(bytes),Time(ms) ./phone_encoder,24612864,1735 Proc,Memory(bytes),Time(ms) ./phone_encoder,24612864,3207 After your change: Proc,Memory(bytes),Time(ms) ./phone_encoder,24608768,205 Proc,Memory(bytes),Time(ms) ./phone_encoder,24592384,490 Proc,Memory(bytes),Time(ms) ./phone_encoder,24600576,1745 Proc,Memory(bytes),Time(ms) ./phone_encoder,24608768,3388 It's actually surprising that your change didn't make it much worse, but compared with the version where the lifetimes are used to avoid cloning anything at all, both of these are extremely slow.