Which is better: performing the prayer on time, or combining it and then performing it again at its original time?

Combining prayers is a permissible dispensation when there is a legitimate reason for it. However, it is preferable not to combine them and to perform each prayer at its appointed time. If refraining from combining prayers would lead to performing a prayer individually, then combining them is more virtuous in order to preserve the merit of congregational prayer. But if prayers are combined by way of jamʿ al-taqdīm (combining and praying earlier) without a legitimate reason, then the second prayer is invalid and must be re-performed.