This summary is not the rule. The rule text controls.
Lawyers have to work on your case with reasonable speed and follow-through. This means they cannot let your case sit untouched for long periods or drag their feet on tasks that need to get done. If there's a deadline coming up or an action that needs to be taken, your lawyer needs to handle it in a timely way.
This rule exists because delays can hurt your case. Missing deadlines, letting evidence go stale, or failing to file paperwork on time can cause real harm, sometimes harm that can't be fixed later. Your lawyer is expected to treat your case with the urgency it deserves, even if they have other clients and cases competing for their attention.
Being diligent also means your lawyer should see your case through to the end. They can't abandon your case partway through or lose track of it because they got busy or distracted. If your lawyer is consistently slow to respond, misses deadlines, or seems to have forgotten about your case, that could be a violation of this rule.
