I wouldn't say Arsenal's performance was poor. This team is strong enough. A few days ago we saw that Arsenal won 3-1 against Liverpool.
However, Arsenal's performance is somewhat erratic. Despite winning consistently, this team is not performing well in every match. Their performance is not completely stable. A few days ago we saw Arsenal fight hard to win against Brentford.
Arsenal face both City and Liverpool in their next 10 matches. And if the team can win these two matches then I believe Arsenal will be champions this season. But winning both these matches is a big challenge for Arsenal.
Despite the fact that the Champions League and the Premier League are two different competitions, I think it was perceivable that Arsenal sometimes struggles with finding solutions in difficult situations like in the game against Porto. Most here (including me) were sure that Arsenal would probably crush Porto or at least avoid going into overtime, since they were playing at home and were so dynamic over the last few weeks. But this is one of the weaknesses they have. A team like Porto should not be underestimated, I agree with that, but Arsenal, if they play their best, shouldn't end up in a coin flip situation like the penalty shootout against Porto. They were about to get knocked out because they were not able to produce an advantage big enough to avoid going into overtime. Losing in Porto and a poor 1-0 at home.
I think that Manchester City is still a mini favorite because they have this little extra. Maybe Liverpool is the same because they are full of passion and literally burning for the title at the moment, as Klopp will leave the club. I would put the chances between these three teams at City/Liverpool/Arsenal = 35/34/31%.