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If by avoiding delay, you mean praying any of the obligatory prayers (in this case Dhuhr) with our Sunni brothers at a time before the prescribed time as understood by Shi'a Imamiyyah fiqh. That will not be counted as fulfilling one's obligation, each prayer has its prescribed beginning and ending time.

The obligatory prayers require the fulfilment of certain demands, as stipulated within the risalah amaliyyah (books of ritual practice) if these demands are not adhered to whilst performing the obligatory prayers then for a follower of Jafari fiqh their prayer  is invalid.

for example the recitation of the basmallah (in the name of Allah the all beneficent the all merciful) before the recitation of a surah (a complete one) during the prayer (except surah tawbah), as an obligation.

Or facing of the qiblah throughout the entirety of the prayer as an obligation (not turning the head from left to right whilst reciting the Salaam) which would invalidate the prayer for a follower of the Jafari school of fiqh.

Or the folding of the arms across the chest instead of holding them by the sides of the body. This would also invalidate the prayer if performed by a follower of Jafari fiqh.

Praying behind and praying with are two different things. There is common a mentioning of joining our Sunni brothers during the prayer, as a sign of unity.

However, one is required to observe the correct time and recite all that which is obligatory oneself, if joining the obligatory prayer led by our Sunni brothers.