Open a message and click the trash icon in the action bar. The next message opens immediately afterwards (Gmail / Superhuman style). Click trash again — next message. This way you can clear 20-30 messages in a few seconds.
Under the hood: Senly collects all these clicks in a batch queue and sends them 220ms later as one bulk request to your provider. No rate-limit issues, no 30 separate API calls that Gmail can throttle.
The same works for archive (archive icon) and mark as spam (spam icon).