Time wasn't supposed to be felt in the lifestream.
At least, that was what Aerith had sensed when she had first entered it. The peaceful, dreaming world that surrounded her in a welcoming blanket of green had washed all her worries away, reminding her that she was safe now and could relax. Aerith knew that couldn't be true, however, because of the clarity that had come with activating her birth mother's dying gift: the holy materia. She had pushed the peace and dream-like state away so she could focus on the tendrils of the planet the lifestream could show her. She had watched in earnest at the struggles and the triumphs of her living friends, relishing that she could see them but disheartening that she was so far away.
And then it had come time, and the lifestream turned from that peace to a state of tautness, like a wound string about to snap. She grabbed this metaphorical string and used the will left in her and leftover from casting holy to win the war against Jenova and Sephiroth. Midgar was in ruins, but the world would survive.
And so Aerith gently let go of the reins and tried to let the beauty of meaningless time envelop her once more. But this time, she found she couldn't. Even when a chokingly familiar presence in the form of her first love, Zack, came to help nudge her to enjoy her afterlife, she couldn't. She couldn't tear her eyes away from the world as it worked to build itself once more. She couldn't stop following the journeys her friends were now partaking, starting new lives and building their hopes and dreams.
Most of all, if Aerith could think like her corporeal self again, she knew she couldn't stop watching him.
The human Cloud Strife had been through turmoil and torture, heartache and loneliness. He had seen the worst of the worst and only deserved the best of the best. He stumbled for awhile as she watched, wishing she could comfort him in his solitude and despair. As the world became poisoned with geostigma and the lifestream twanged painfully at the creeping sickness taking more and more life, she felt as if she could cry an entire river as she watched him wrap new bandages and hide his arm under a canvas of black fabric. The style suited him, but the suffering did not. And the fact that he was staying in the old church in sector five, hoarding materia and openly showing emotions hurt her all the more.
So Aerith did the unthinkable: she persuaded the lifestream to allow her to communicate. The lifestream's gentle peacefulness was wary at the idea, but it eventually decided to make a special exception because it was Cloud Strife, the hero of the planet. And so, she opened up that chain of communication, waiting for him to reach out. And he did, many times. He blamed himself for her death, and that broke her heart. The small gifts she gave to him and the planet were enough to heal the geostigma and bring hope back into his heart. And when her time was up and Zack was waiting for her again, she willingly went this time, knowing time would hopefully be on her side now.
And yet, time still passed painstakingly slow, as if it was aware that she was watching and waiting for the moment eternity would be hers. She kept an eye on Cloud and Tifa and their small, makeshift family. She coaxed flowers to the dirt outside the front of the bar. She grew meadows at the cliff where Zack died that Cloud loved to visit. She tended to her flowers in her church, now surrounding a bright, full pool of perfect water. She gave them warm breezes and sunny days, she sent them every good will that the planet had to offer. Like a silent guardian angel, she watched over them, and every time he took notice of her presence, Cloud would smile, bow his head and fold his hands together, and thank her.
Finally, the time she had been waiting for came. The pleasant, soft green parted as if it was a large curtain, and suddenly Cloud had joined her. He had lived a long, full life with hope in his heart, and he told her it was because of her. He was now warm and love and light, the best of things she had always seen in him. Eternity raced in her veins and love seared in her heart, each fiber of her being promising to lead him into the peaceful eternity he deserved. Taking his corporeal hand, she pulled him close and kissed her love into him, welcoming him to forever.
Eternity was theirs now.