> Second, my sense--this isn't something I have data for--is that the code that is output by compilers can be further optimized much much more than for x64. One example is library routines not using all of the available hardware algorithms, e.g., in cryptogrpahy.

Your senses are not failing you. Let's pick apart a specific example being the stock openssl binary (/usr/bin/openssl) compiled with unknown C compiler flags vs the manually built one (I have used the one that homebrew ships).

The stock binary reports:

$ /usr/bin/openssl speed sha256
Doing sha256 for 3s on 16 size blocks: 24823543 sha256's in 2.99s
Doing sha256 for 3s on 64 size blocks: 17875138 sha256's in 2.99s
Doing sha256 for 3s on 256 size blocks: 13158887 sha256's in 2.99s
Doing sha256 for 3s on 1024 size blocks: 5565350 sha256's in 2.99s
Doing sha256 for 3s on 8192 size blocks: 874073 sha256's in 2.99s
LibreSSL 2.8.3
built on: date not available
options:bn(64,64) rc4(ptr,int) des(idx,cisc,16,int) aes(partial) blowfish(idx)
compiler: information not available
The 'numbers' are in 1000s of bytes per second processed.
type 16 bytes 64 bytes 256 bytes 1024 bytes 8192 bytes
sha256 132632.74k 382026.73k 1125063.44k 1902869.91k 2391966.26k

Let's tinker with the homebrew's CFLAGS and add ARM64 v8.4 specific optimisation flags + unroll loops + free up the frame pointer register for the general use (-Ofast -funroll-loops -fomit-frame-pointer -pipe -w -pipe -march=armv8.4-a+simd+crypto+i8mm+bf16+fp16):

$ /opt/homebrew/opt/openssl@1.1/bin/openssl speed sha256
Doing sha256 for 3s on 16 size blocks: 76652798 sha256's in 2.99s
Doing sha256 for 3s on 64 size blocks: 52778508 sha256's in 2.99s
Doing sha256 for 3s on 256 size blocks: 22320676 sha256's in 3.00s
Doing sha256 for 3s on 1024 size blocks: 6744512 sha256's in 2.99s
Doing sha256 for 3s on 8192 size blocks: 899002 sha256's in 2.99s
Doing sha256 for 3s on 16384 size blocks: 451670 sha256's in 3.00s
OpenSSL 1.1.1m 14 Dec 2021
built on: Tue Dec 14 15:45:01 2021 UTC
options:bn(64,64) rc4(int) des(int) aes(partial) idea(int) blowfish(ptr)
compiler: /usr/bin/clang -fPIC -arch arm64 -Ofast -funroll-loops -fomit-frame-pointer -pipe -w -pipe -march=armv8.4-a+simd+crypto+i8mm+bf16+fp16 -mmacosx-version-min=12 -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX12.sdk -DL_ENDIAN -DOPENSSL_PIC -DOPENSSL_CPUID_OBJ -DOPENSSL_BN_ASM_MONT -DSHA1_ASM -DSHA256_ASM -DSHA512_ASM -DKECCAK1600_ASM -DVPAES_ASM -DECP_NISTZ256_ASM -DPOLY1305_ASM -D_REENTRANT -DNDEBUG -isystem/opt/homebrew/include -F/opt/homebrew/Frameworks -isysroot/Library/Developer/CommandLineTools/SDKs/MacOSX12.sdk
The 'numbers' are in 1000s of bytes per second processed.
type 16 bytes 64 bytes 256 bytes 1024 bytes 8192 bytes 16384 bytes
sha256 410182.20k 1129707.19k 1904697.69k 2309826.18k 2463085.08k 2466720.43k

By simply specifying ARM64 v8.4 specific code generation flags, the speedup for the 16 and 64 byte blocks is nearly 4x, is nearly 2x for the 256 byte test and a 21% speedup for the 1024 byte test.

What I have begrudgingly noticed, though, is that way too many open source packages go a very long way to forcefully override CFLAGS / CXXFLAGS environment variables (I set mines in the shell rc file) to set them to something else. What are the most usual / typical choices, though? Not very imaginative: «-O2 -g», «-Ofast/-O3», «-O -g» or simply «-O». My personal plea to open source maintainers: can you please stop overriding CFLAGS and CXXFLAGS variables in your Makefile's / CMakefiles unless you know that a specific combination of the optimisation flags breaks your package. «make» / «cmake» et al allow for the conditional or additive setting of variables (i.e. ?= or += vs :=), so please stick with those and inherent the values when they are present in the environment.