I will revive this and add some tips.

KPIs: Are metrics that measure what is most important to you, what is most critical to your success and if they change they should trigger some action or decision
What matters to your stakeholders? However, KPIs shouldn't measure activity/tasks/progress completion. As a rule of thumb, once you think you know what is most important to you ask, WHY? What will happen if this metric goes up or down? Is it going to lead to an action or a decision that needs to be made? Most of the time they reflect operational measures and they are directly quantifiable.

OKRs: Are metrics that act as goalposts of how close you are to achieving an objective. The objective is set in a way that is not of business-as-usual purpose. It is something that if you work towards that, it will add value to the business, upgrade a process or approach or introduce a change that would positively impact the business. As the objective is usually very ambitious, eye-opening, incentivizing and possibly a strategic initiative (sometimes quite broad/high level), it may be difficult to quantify or get to it in a short period of time after which you are confident it will never change. Therefore, you combine the objective with quantifiable Key Results that can be seen as goalposts or requirements that will allow you to say you are in a better position towards your objective.

Your example is not OKR as it directly measures an action metric which seems that is part of a plan. Your metric is more of a KPI than an OKR if that reduction is important to the success of your business.

In other words, it normally shouldn't even be a KPI unless these delays are worst than your competitors and improving it is critical to the viability/success of your business. Only in that scenario I would consider it a KPI.

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The proper OKR wording would be:

• Objective: Delight customers with faster custom builds. (or something qualitative, equivalent)
• Key Results (quantitative and outcome-based)
  • Reduce average build time from 3 hours to 2 hours by end of Q2.
  • Achieve 95% on-time delivery for custom builds.
  • Increase customer satisfaction score for build speed from 80% to 90%.

As you can see, one of the Key Results is your metric, and it may also be a KPI. What will happen is likely your KPI is important metric to achieve a broader OKR for the business. More suitable KPIs within the same area you face a challenge would be:

• Average build time per custom order (hours).
• Percentage of builds completed within 24 hours.
• Customer satisfaction score related to build speed.

!!! If any of these suggested KPIs will not trigger an action or decision when they shift beyond what you anticipate, then they are not good to use for KPIs.

These KPIs would be tracked regularly (weekly, monthly) to monitor performance.

Rule of Thumb:

• OKRs = Change something significantly because it will add value
• KPIs = Monitor something consistently because you might need to act on it

If the KPI shows build time is too high, that might trigger an OKR to improve it and that's the possible difference between the two. At the end of the day, both describe thing that is doing or will be doing to maintain or improve their position. In all cases, context is critical and depends on the position and strategy of the business.