Drip "Waits" of less than an hour

By "were sent at the same time", do you mean that you have a Send Email flow action after the Wait flow action?  If so, you may be experiencing the output of the Send Email flow action being queued by the Marketo email server.  You could add a Change Data Value flow action between the Wait and Send Email steps and have it update a field with the {{system.DateTime}} token, so you can see if the leads are leaving the Wait step when desired and it's just a queuing delay that is cauing the emails to be batched together.

Sounds like a busy campaign queue to me. Having data changes in between will stretch it, as the priority of data changes is lower than sending mails. You can only run 2 processes at once. The rest is queued (inlcuding data changes, triggers and so on). If you don't mind it to be 10 or 15 minutes, add a random data change step in the flow before each wait step. If timing is critical you should think about cleaning up your entire Marketo instance (deactivating old triggers). Simplyfying scoring, interesting moments and so on). Ask support to help you with this.