Is one who intentionally delays the Dhuhr and Isha prayers, and prays their preceding Sunnah (optional prayers) after their usual time, sinful, knowing that he does so in order to pray at a leisurely pace, and before its time expires?

The time for the pre-prayer voluntary (nafilah) offering is before performing the obligatory prayer. It is not mandatory to perform it before the iqamah (definition) (call to commence prayer) in the mosque. The prayer time is extended, and it is a flexible obligation (wajib (definition) musawwa'), meaning it is permissible to perform it in any part of its time. Therefore, there is no harm in slightly delaying the Dhuhr (noon) and Isha (night) prayers, as long as the prayer is performed within its prescribed time.