I have been advised that too many commas "slow things down" and that I should leave them out whenever I can. I tried that and an agent I queried returned my sample with the note, "punctuation needs work." Was I wrong to take that advice?
Probably. Usually, when folks give this advice they are referring to the "Oxford" commas, or the last comma before "and" in a series. Some people even leave out all commas for things in series, but editors shudder when reading a manuscript like that, knowing they will have to work hard to put them back in or face the phrase "could have used more editing" in future reviews.
Even deleting an Oxford comma can result in moments of embarrassment, as when I waited tables in my grandma's restaurant folks regularly ordered "pie and sherbet" because there was no comma on the menu to show they were two different desserts.
Here's my best very basic advice about commas. Never confuse your reader. Don't take them out, just because someone says you shoud. They are tools of language and are there for a reason. Always put them in if there's a reason for them to be there, like where you'd pause for breath or effect. There's a world of difference between:
"Shoot John!" and
In one sentence John gets shot, in the other he is instructed to shoot. That can make a big difference to your reader, as well as to John.