You don't mention brand of the switch, any management software, or any configuration (in particular how that interface was previously configured), so I can only answer generically.
Most default switch MAC address tables have a relatively low timeout to age out entries. Just because you are only seeing 19 devices from the same VLAN on a port currently doesn't mean that there aren't other devices that will need to utilize a different VLAN on the trunk.
You say you "guessed that there was a switch connected to that interface." Guessing is a bad way to decide on making changes in a network. This indicates you do not understand the network in enough detail to be making the change and you should spend more time investigating before doing so.
Is that wrong?
Connecting two switches utilizing access ports? Wrong, no. It is considered a best practice to connect two switches with trunk/tagged ports. However, there are circumstances where using access ports is perfectly fine, if not necessary.
There may also be other considerations. For instance, there may be default configuration (or other configuration on the port) that could apply differently to access ports or trunk/tagged ports. For example, if your switches use VTP (or a similar mechanism), VTP only runs over trunk/tagged ports. Another example would be spanning-tree portfast (or similar), which can be applied by default to access ports along with features such as BPDU guard.
Wrong to make a change based on a guess? Yes, absolutely. Unless you have some pressing reason forcing your hand, you shouldn't make decisions in networking based on guesses. You are likely to create issues.
Does that worsen the switching perfomance?
No. There should be no noticeable difference in performance between an access port and trunk port. However, look a couple paragraphs up as there may be configuration that is applied differently to access ports and trunk/tagged ports. This configuration may have some impact on the port operation.
Should I go to talk to the person who manages the other switch to know how he configured the interface connected to the switch that I manage?
Yes and you really should have done so before making the change. When other parties are involved, it is almost always best to reach out to them before making any changes.
Reverse the situation, what if the other person were to make such a change without informing you? Go a bit further and say this disrupted operation on your network in some fashion. Wouldn't you prefer that you had been informed before hand so you wouldn't have to spend time and effort troubleshooting an issue created by someone else?