For certificate issuance, we have internal Service Level Objectives (SLOs), but no agreements or guarantees.
When our service is operating normally, you can expect issuance in 2-10 seconds, plus any time required on your side to get ready to answer the ACME challenge. Staging and production often have very different performance if you're comparing milliseconds, but should share that 2-10 second estimate.
We always recommend you design your service so that it can gracefully handle failures or delays on our end.