I've been thinking about this a lot lately.
For me, it used to be back-and-forth emails trying to get simple approvals. A client would ghost for a week, then come back with a dozen comments on the first draft instead of giving feedback in stages.
I fixed it by setting up a simple Google Sheet with a "status" column for each deliverable - draft, in review, approved. Clients update it directly and I get notified. Cut my follow-up time by about 60%.
What's the one thing that wastes the most of your time on client projects? And have you found a system to fix it?