Well, you can also think about it this way.
Depending on the location you are from, you can have a who new dialect or sub-language category. My girlfriends grandparents were born and raised in Mexico and moved here 40 years ago (they are 80 now). The language they use is far from proper - as related to learning Spanish in the classroom. The town they came from has different variations on words, phrases, and sentence structure. So, even when someone else speaks, they can take a second to try and put together what they said.
There is a common Spanish language that is used, but there are many unique parts to where you come from. I can only imagine this is the same for someone from Spain. I would think they will be able to understand each other, but it might take some thinking at times.
A good example is the Spanish language in Puerto Rico. My girlfriend's father, who is the son of the grandparents mentioned, is 100% Mexican and speaks Spanish and English both fluently. I asked him why it is so hard to understand someone from Puerto Rico, and he said it is because they adapted their own signature language. Most, if not all, of their words are shortened and spoken at a much higher rate.
I know that is a bit to read, but hope it helps
