I should've done this a while ago
Anyways, this has always been a problem when it comes to fanfiction.
You shouldn't have everything be fast, but not everything has to be stretched out. Slow stories get extremely boring, especially if it was fast paced at first but then came to a halt and slowed down.
There's been many offenders to this, but all I have to say is;
Not everything has to be a slow burn. Write a cute relationship, have the relationship be a character arc unless it's a forbidden love type of thing and in order to get together they have to dodge obstacles.
Also, this blends in with the story thing, but things shouldn't drag on. Magia Record and Beastars (manga) did this, Magia Record being more obvious in this.
Beastars (the manga) has an arc that sort of drags but Melon picks it up and the original pass continues.
Some arcs can be short, others can be long. You've got to understand what makes what.
Find a middle ground, add fluff but not to much, add angst but not to much, not everything has to be packed with story.