9 nights in Spain

As much as I loved Sevilla, with 10 days/9 nights I would stick with Barcelona and Madrid, with most of that time in Barcelona.

Two full days per city (average) plus travel days seems very rushed to me. I'd save Sevilla (and Cordoba, Granada, Ronda, etc) for another trip, preferably not in summer. At the "Explore Europe" link on this website, click down to Spain and then the cities to see what our host recommends for each.

You’re going to need a minimum of three nights each in Barcelona, four in Madrid and two in Sevilla. Furthermore, I recommend being organized so you don’t waste time. This is a hard question to answer because I don’t know what you want to see, but I highly recommend the following:
Barcelona – Picasso Museum and the Sagrada Familia and make sure you purchase tickets for the church before leaving home. I would also take Rick Steves’ self-guided walking tours and also in Madrid and Sevilla.
Madrid – Royal Palace and the Prado Museum that will take a full day. I also suggest a stop at the Centro de Arte Reina Sofia to admire Picasso’s prized masterpiece the Guernica. If there’s one place to take a day trip from it’s Madrid and go to Toledo.
I feel Sevilla can be seen in two nights. You may want to take in a flamenco show one night while there.

Another vote for dropping Sevilla. I’d give 5 days to Barcelona and the rest to Madrid, assuming you’ve never been to either. Remember, one of your days will be a travel day.

I love Sevilla, but save that for a trip when you have time for Cordoba and Granada as well. There is so much to do in Barcelona, that I would suggest up to 7 nights there unless you have plans to do some day trips from Madrid. In any case, I would spend more time in Barcelona than Madrid. Barcelona also has a number of wonderful potential day trip opportunities.

I would do either:

4 nights Madrid, 2 nights Cordoba , and 3 nights Sevilla.

or

4 nights Barcelona, 2 nights Zaragoza, and 3 nights Madrid

After touring Portugal a few years ago we flew to Madrid for 4 nights, & did a day trip to Toledo. We took the train to Barcelona for 4 nights. Wish we had visited Girona. I would agree to delete Sevilla this time.

Is it too much? That's a tough question to answer. Can you do it? Absolutely. Should you do it is an entirely different question which really depends upon three factors - what you want to see/do and, what your "travel style" is and what time of year you are traveling - Sevilla gets incredibly hot in the summer and will slow your pace. If you simply want a glimpse of each of these cities, then it might work for you - but that is all you'll get - a walking tour of each city and two, possibly three sites - a good appetizer but definitely not a full meal - LOL!! But if you want to "dig a bit deeper "or want a slightly less frantic pace, then I would limit it to Madrid and Barcelona - there is more than enough to see/do/eat! in both of these cities to easily fill 10 days. If you find yourself at loose ends with either city there are day trips from each such as Toledo from Madrid and Girona from Barcelona
On my first trip to Spain, we spent 10 days in Madrid and Barcelona - including Toledo and El Escorial. We weren't bored in the least - and I will go back and dig deeper into both those cities.
Don't know if this helps but there are my two cents!

If this is going to likely be your only trip to Spain then visit all three.

Before I visited Spain for the first time almost 40 years ago, I read in a guidebook that two cities are a MUST in Spain:
Toledo and Segovia.
After visiting much of Spain, I would agree with this advice.

I love Barcelona and hate to eliminate one of the three, but I would not cut Sevilla.

Plan on visiting Granada as well at some time in the future.