It really doesn't make sense to optimize anything in a bootstrapping compiler. Usually the only code that will ever be compiled by this compiler will be rustc itself. And rustc doesn't need to run fast - just fast enough to recompile itself. So, the output also probably won't have any optimisations applied either.
Where optimizing for size is optimizing for speed is when it's faster (in terms of wall clock time) for a program to compute data than to read it from memory, disk, i/o etc, because i/o bandwidth is generally much slower than execution bandwidth. That means the processor does more work, but it takes less time because it's not waiting for data to load through the cache or memory.
int main() { for(;;); }
Eg, quicksort vs bubble sort. Quicksort is usually more code but faster.
Or a linked list vs a btree. The linked list is less code, but the btree will be faster.
Or substring search, with or without simd. The simd version will be longer and more complex, but run faster.
Even in a for loop - if the compiler unrolls the loop, it takes up more code but often runs faster.
If you have two programs that both do the same work, and one is short and simple, I don’t think that tells you much about which will run faster.