"at times and it slowed"
"can be tricky and I sometimes struggle"
Granted, you might want a dramatic pause or the comma might break up parts that could be misconstrued as together. Chiding Alex with superfluous commas? Generally speaking I recommend dropping commas where you can. Like you said, they're speed bumps.
There's no need for either of us to be snarky. I told Alex that I enjoyed his writing and respectfully offered feedback.
Commas can be tricky, and I sometimes struggle with them as well."
I'm not saying you're wrong. Yes the sentences are complete but how self-sufficient are they? It's a sliding scale from no comma to comma to period. Some of these rules are not hard and fast. The art of writing is slowly evolving and to a degree it's a matter of taste. Some people love hyphens. In this case there's a strong enough correlation, conceptual link between the sentences such that the reader doesn't need a comma to read what you're saying in one breath. If it sounds good in one breath, that's a strong signal you don't need a comma. Knowing the basic rules is just the beginning. I'm dead serious.