user_id_tp     = tutor_profiles.index_by(&:user_id)
tutor_subjects = TutorSubject
  .where(tutor_id: user_id_tp.keys)
  .group_by(&:tutor_id)
  .to_h { |uid, subjects| [user_id_tp[uid], subjects.map(&:subject_id)] }

# Faster

user_id_tp     = tutor_profiles.index_by(&:user_id)
hash_of_arrays = Hash.new { |h,k| [] }
tutor_subjects = TutorSubject
  .where(tutor_id: user_id_tp.keys)
  .each_with_object(hash_of_arrays) do |subject, subjects|
    subjects[subject.tutor_id].push(subject.id)
  end

# Fastest

user_id_tp     = tutor_profiles.index_by(&:user_id)
tutor_subjects = TutorSubject
  .where(tutor_id: user_id_tp.keys)
  .pluck('tutor_id', 'subject_id')
  .each_with_object(hash_of_arrays) { |(tid, sid), subjects| h[tid] << sid }