The five divine beings were caught off guard and could only watch him go. When they realized that something was wrong, he was already gone.
When Loki entered the River of Time, he realized that something was different this time around. What was different was that he felt bloated and heavy.
His mind felt sluggish, so he couldn't control where to stop. He could only allow Fenrir to go upstream until Fenrir had to stop or got too tired to go further. This was because his Mental World was too busy digesting the mountain in his mind.
When he had tried to fuse with the fragment of a dead heart, it had taken him more than an hour to fuse with it and use it to advance to rank 5. But things were more difficult than that this time around.
This time around, he is fusing with an entire heart, and the heart isn't dead. The heart is only half dead. So one can imagine the difficulty of fusing with it.
