Syncthing will use local connections whenever possible. Make sure that you don’t have a firewall blocking direct connections somewhere. Also, if this is Windows, the network connection needs to be set as “private”.
The ignore patterns are relative to the folder path in Syncthing. If you want to ignore a specific folder with everything inside it, just add its path to the patterns, e.g. /dir1/dir2. On Windows, you can also use the backslash \ as the path separator.
Just to give a more concrete example, if I was using the default Syncthing folder located at C:\Users\User\Sync and wanted to ignore a folder located under C:\Users\User\Sync\Dir1\Dir2, then I would add /Dir1/Dir2 (or \Dir1\Dir2) to the ignore patterns.
Im not sure, but I think if in the folder Dir1 a file named Dir2 it would be also ignored, so in this case may better to use /Dir1/Dir2/ with / at the end if the file is want to include for sync. Right?
Not really, as it is impossible to have a folder and a file with the same name in the same path. The trailing / would be needed only if you do want to have the directory created, but everything inside it ignored.
There could be a situation though, where you would want to ignore only folders, and not files, located in different places. Then using a pattern like folder/ could probably be useful.