Why did Lord Vishnu grant 7 janams if Jai Vijay

Hello! Destruction is always easier, and goodness is always hard. The path of Adharma is temporary happiness, but it results into life long pain. Dharma, though, gives temporary pain and life long happiness. And building a good deed can take a life time, but that same deed can be destroyed in seconds. The Lord tests his devotees to see whether his devotees are worthy of his live, while Adharmis need to be killed quickly. I hope this helped!

But isn't it unfair that the devotee have to suffer the pain of separation from the Lord longer compared to the enemy?

In that case why would anyone want to become a devotee, if the fastest way to be near the Lord is becoming his enemy ?

This is something that's been bothering me ever since I watched that episode. What could be the reason behind this ?

Evil can not forever continue harming others. It has to end & the medium to finish, depends on the intensity of Evil. If the Evil is very powerful then God himself appears or incarnates.

Jay and Vijay did a mistake by insulting the Sanat Kumar's and hence as a punishment had to bear separation from the Lord... Now your question that y 7 janams as devotees and 3 as enemies... Well though none can truly understand the ways of the Lord but just conveying my thinking... When one is a devotee of the Lord one is still connected to the Lord ... Even if not in a physical form but yet at a sub conscious level there is oneness between the God and the devotee so the punishment was less... But in case of being enemies they would even forget their aradhya and hence not even be able to have Shri Hari in their hearts ... So their punishment would be more intense and hence over sooner...

Now to your other question as to y is it easier to be an enemy and y a devotee has to face so many hurdles... True outwardly Ravan or maybe Duryodhan lived a very lavish life throughout their lives and in the end met their death at the hands of Shri Hari... So it appears to be a win win situation for them ... While the Pandavs throughout their lives suffered and even after war had to rule their kingdom with the ever present burden of guilt on their shoulders ... I also wondered the same for a long while... But truthfully speaking the greatest enemies of anyone are not outside but inside... The six vices of greed, jealousy , selfishness, lust, pride and excessive attachment are the enemies which attack from within your heart and make it as hollow as a rotten log of wood...outwardly even though u have everything actually speaking u r never happy on the inside... And truly it's a very bad feeling... On the other hand the devotees of Lord can find happiness even in the worst of situations... Like the Pandavs no matter what the situation were always happy and contented in life taking Shri Krishna's name...