Whenever we use an adverb related to time,* we are telling.
This isn’t always bad, and it’s certainly unavoidable. You want to mention time in your writing. Some sense of time can ground a person into a story, especially if the story plays with time, or your sense of time, as a theme or trope. However, we don’t typically think in terms of time.
By that, I mean, yes, we do think “In two weeks I’m getting married,” but we don’t think about the passage of time as it’s passing. In our day to day lives, time just…passes. So a construction such as,
I grab my keys and dart out the door. Five minutes later, I stand at the bus stop.
is artificial when it comes to the lived experience of the character.
That’s because we live those five minutes. In reality, I grab my keys, dart out the door, run down the steps, turn right, go a block, hop over the trench worn by truck tires in the faded asphalt of the alley, etc., and I’m thinking more about navigating my way to the bus stop than “fifty-seven, fifty-eight, fifty-nine, one minute.”
But you can’t write a good story without “five minutes later,” can you?
So yes, sometimes telling is important. You don’t need to show every detail of a person’s life! Much of a person’s life is going to be trivial or boring. Why waste your word count on it?
Simultaneity, on the other hand, is fairly infrequent, so words such as as should see limited use in a story.
I fell out of my chair as he bumped past me.
Usually, in cases like this, we’re looking at cause and effect. People don’t fall out of their chairs for no reason. If you fell out of your chair because he bumped you, then the sequence of events is this: He bumped me. I fell out of my chair. It should be written that way.
When you unnecessarily add the word as, you are dipping into narrator-space to tell the reader that two events are happening simultaneously when they’re not.
Now maybe one thousand things happen to us each day. Maybe ten of those things are true coincidence: unrelated things happening at the same time. The cat, prowling on top of the kitchen cabinets, knocks a vase down just as you’re walking past. Totally unrelated. But the vase lands on your head. It happens, and coincidences happen to all of us, but ten out of one thousand is a one percent chance. Because totally spontaneous coincidences happen rarely, you should reserve your use of as for those specific things.
Telling can be necessary, but telling always removes us from the heads of the point of view characters. Reserve your telling words, like your filter words, for the times when you must use them. It makes your writing look deliberate and professional, and will cut down on the amount you pay your editors.
*Soon, once, yesterday, tomorrow, later, and other such time-related words are all adverbs.