replace as well as replace! do not work properly if count is supplied:

julia> replace(ChainedVector([[1,2],[1,2]]), 2=>20; count=1)
6-element ChainedVector{Int64, Vector{Int64}}:
  1
 20
  1
 20

julia> replace([1,2,1,2],1=>20,count=1) # expectation
4-element Vector{Int64}:
 20
  2
  1
  2

The underlying code just applies the count limitation to each chunk.

Sprinkeling @inbounds on the array get/setindex didn't improve performance, but I should probably add it.