Hi @nofiat :)
Yeah, the small delay is by design - first, steem blockchain does not allow to make 2 transfers within 3 seconds, so if @tipU is busy (sending other payout, confirmation etc.) the payment got delayed.
Also there's some randomness added to evenly distribute payouts trough the whole day.
As for reinvest - first I need to deal with one investment of 2500 SBD from a single user - this "used" a large part of the @tipU SP :(
Seems a good problem to have... I look forward to giving you similar sized problems soon enough :)