"It's probably the worst bit of today," said Alonso of the large time deficit.
"It's not the seventh place or how far we are from the podium, it's that 1.8s is a lot and we want to change this.
"What we need to do is just work and deliver the results. There will be new parts in Canada, that's for sure, and we will bring some to Monaco and Silverstone, we brought parts here, we have six or seven, but it's difficult to make magic."
Alonso denied that Ferrari's upgrades for Spain had turned out to be a retrograde step, saying it was more a case of Mercedes' developments being even better than its rivals'.
"I think they brought some new parts which seem to work fine, and they had a big gap [already], now maybe they've extended the gap,"he said.
"We brought some new parts, some of them were positive, some of them negative, and we will need time to develop, but the car is definitely better than in China.
"This is always the same thing, you bring a couple of tenths and the other teams bring a few tenths and you are in the same position. They bring less, they gain some, you bring less, the gap increases... it's nothing new this year."
The Ferrari driver also suspects the low-grip conditions all teams have encountered at Barcelona are exacerbating the pack behind Mercedes' handling shortcomings.
"We faced difficult conditions all weekend, not only for us for everybody with low grip, and maybe that was an extra factor to increase their advantage," Alonso suggested.
"I'm confident, or I hope, that this 1.8s will not become that much more. We will close that gap starting from Monte Carlo, there are not many straights and hopefully we can be closer.
"From Canada onwards we will try to be not that far away."
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