For bigger instances it’s very slow to process custom ranges on the fly. However, Piwik gives possibility to choose some custom ranges as default dates after logging in (i.e. last7, previous7 etc). It would be a nice feature to allow pre-processing for those specific ranges during daily archiving. Also it would be nice to be able to define other custom periods to be pre-processed, so having knowledge what they are user can use them smoothly even on big instance.