I made a benchmark project to compare inlinable version vs non inlinable.
The results are following:
name time std iterations
----------------------------------------------------------------------------------
Encode (no inline) 1054396.000 ns ± 3.25 % 1286
Encode (inline) 420666.500 ns ± 3.57 % 3292
Decode (no inline) 820250.000 ns ± 3.20 % 1689
Decode (inline) 466583.000 ns ± 3.63 % 2972
Decode json with ZippyJSONDecoder (no inline) 246625.000 ns ± 5.70 % 5568
Decode json with ZippyJSONDecoder (inline) 176833.500 ns ± 5.75 % 7798
So there are clear differences between versions. But I also should mention that forcing the compiler to actually inline everything is quite tricky and requires a lot of attention from all parties: stdlib, the encoder/decoder vendor and the consumer. But at least it's possible.
One of the most tricky thing is that I have to keep signatures of the encode(to: Encoder) and init(from: Decoder) to not break ABI. So they are not generic and all of the inlining magic around them is kinda fragile.