Spain and Porto in 9 nights 10 days

With only 9 nights, you really only have 8 days since this includes travel days
- too short to add a third city like Porto since the only efficient way to get there is to fly. There is more than enough to see/do in both Madrid and Barcelona - especially if you add in a day trip or two.

Thanks Nancy! Do you recommend visiting Malaga or Seville instead since it can be easily accessible by train. Only reason why I ask is because others have told me 2-3 days in Barcelona/Madrid each is more than enough.

With only 9 nights, I would not try to include Porto. You would need to fly and waste a lot of time doing that.
Split your time between Barcelona and Madrid. Do day trips from Madrid to Toledo and Segovia.

2 days is not enough for Barcelona and 3 days is tight; more comfortable to spend 4 nights in Barcelona, 4 in Madrid with day trip to either Toledo or Segovia (or both, if you think you can visit Madrid in a day, which you can if museums aren't your passion), and a final preflight night in Barcelona.
Or another idea : head straight to Madrid on arrival day in Barcelona (if flying transatlantic without stops you will be in Barcelona early enough); yes, it will be exhausting, but you can sleep on the train, and you will waste less time in hotel changes.

(tip for next time: fly "open jaw" next time, into Barcelona and out of Madrid or vice versa)

With only 8 days, I would focus only on Madrid and Barcelona. There are a number of day trips (Toledo, Segovia, etc., etc.) that work and thus maximize your time on the ground and each city had more than enough to interest you. Every change of location will take betewen 1/2 tto 3/4's of day even if you aren't flying!

Arrival day: even if you land in the morning, it can take you an hour or two to get through the airport, another hour to get to your hotel and drop your luggage. You are likely to be sleep-deprived and jetlagged, so your first day will be a hazy struggle to stay awake and you won't absorb much. Departure day: you have to be at the airport 3 hours before departure and you'll want to alow an hour to get there. Unless you have an evening flight, you have no time to do more than a lesiurely breakfast and/or a short stroll before picking up your luggage and leaving Barcelona. That's why you have 8 days, not 10.

I don't know who told you 2-3 days is enough in Barcelona. You could easily fill your entire time just in the city and there are several great day trips too. Browse this site. OTOH I would agree that Madrid doesn't need more than a day or two, unless you enjoy European painting, in which case you'll want 2 full days just for the 3 major art museums. And as others have pointed out, there are good day trips too.

All in all, since you are locked into R/T Barcelona, don't waste a half-day of travel to another city and another half-day getting back to Barcelona. That's 3 hotel changes and a full day wasted. I've spent over 14 full days in Barcelona in 3 visits, and the only sight I can remember revisiting is MNAC, a great art museum and that was because I didn't allow enough time there on my first visit.