Overloading Base.hash is 2x slower than an identical function

General Usage Performance
1

Can anyone explain (or reproduce) the following?

Why is the Base.hash version slower and why the allocation?

julia> using BenchmarkTools

julia> mutable struct Foo end

julia> struct Bar
           f::Foo
           x::Int64
       end

julia> Base.hash(b::Bar, h::UInt64) = hash(objectid(b.f), hash(b.x, h))

julia> Base_hash(b::Bar, h::UInt64) = hash(objectid(b.f), hash(b.x, h))
Base_hash (generic function with 1 method)

julia> b = Bar(Foo(), 1)
Bar(Foo(), 1)

julia> @benchmark Base.hash(b, UInt64(0))
BenchmarkTools.Trial: 
  memory estimate:  16 bytes
  allocs estimate:  1
  --------------
  minimum time:     32.773 ns (0.00% GC)
  median time:      35.294 ns (0.00% GC)
  mean time:        37.063 ns (0.32% GC)
  maximum time:     448.193 ns (91.04% GC)
  --------------
  samples:          10000
  evals/sample:     991

julia> @benchmark Base_hash(b, UInt64(0))
BenchmarkTools.Trial: 
  memory estimate:  0 bytes
  allocs estimate:  0
  --------------
  minimum time:     14.408 ns (0.00% GC)
  median time:      15.383 ns (0.00% GC)
  mean time:        15.637 ns (0.00% GC)
  maximum time:     55.585 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     998

julia> @which Base.hash(b, UInt64(0))
hash(b::Bar, h::UInt64) in Main at REPL[4]:1

julia> @which Base_hash(b, UInt64(0))
Base_hash(b::Bar, h::UInt64) in Main at REPL[5]:1

julia> versioninfo()
Julia Version 1.6.0
Commit f9720dc2eb (2021-03-24 12:55 UTC)
Platform Info:
  OS: macOS (x86_64-apple-darwin19.6.0)
  CPU: Intel(R) Core(TM) i5-8259U CPU @ 2.30GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, skylake)
2

Ah. Interpolating is the cause:

julia> @benchmark Base.hash($b, UInt64(0))
BenchmarkTools.Trial: 
  memory estimate:  0 bytes
  allocs estimate:  0
  --------------
  minimum time:     8.727 ns (0.00% GC)
  median time:      8.989 ns (0.00% GC)
  mean time:        9.292 ns (0.00% GC)
  maximum time:     89.672 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     999

julia> @benchmark Base_hash($b, UInt64(0))
BenchmarkTools.Trial: 
  memory estimate:  0 bytes
  allocs estimate:  0
  --------------
  minimum time:     8.727 ns (0.00% GC)
  median time:      8.778 ns (0.00% GC)
  mean time:        9.336 ns (0.00% GC)
  maximum time:     71.057 ns (0.00% GC)
  --------------
  samples:          10000
  evals/sample:     999