To fix that, you have a few options.
Split a big file into smaller files
This is the most effective solution. For instance, if you have a KML file with every campsite in Europe, Maps Pro will by default load every single campsite and plot it on a map. If you want to look for travel options in France for instance, you don't need to load the campsite in other countries.
Having a file with fewer points will make it load a lot faster into Maps Pro.
Check first that the website you downloaded your file from don't already provide smaller files to download (one per country for instance).
You can also split the file manually. To split the file, make a copy of the original, open the file with a text editor such as Notepad++ (free) and delete all the parts of the file that you don't want. Save the file and open it in Maps Pro.
Only show the overlay at a certain zoom level
If you can't split your big file into smaller file, you can use this other solution to achieve better performances. With Maps Pro, you have the option to show the overlay at a certain zoom level, for instance when zoomed in an area where you want to look for a camping spot.
By default, you will notice that Maps Pro zooms on the overlay when you select it. So even if you are zoomed in on an area that you want to work on, Maps Pro will zoom out and show all the POI, which can be unexpected sometimes.
To fix that, just go to Menu > Overlays > Add GPX/KML/... files > Click your overlay on the list and some buttons will appear at the bottom. Click Options > Toggle off "zoom when opening".
Now Maps Pro won't zoom on the overlay when you toggle it on and off.
Toggling off "Show labels" can also speed up the application.
Convert your file to the GeoJSON format
When using KML / KMZ or GPX files, Maps Pro has to read the file and read the information inside of it (names, coordinates, etc.). Once the file is read, Maps Pro stores it in memory as a GeoJSON file.
When using big files, reading the file can take Maps Pro a few seconds. You can save these precious seconds by converting your file to the GeoJSON format beforehand.
Online tools such as Ogre (http://ogre.adc4gis.com/) will allow you to convert your file to the GeoJSON format. The resulting file will load quicker but some information might be loss in the process, especially for complicated KML files.
Still having issues?
If you have more issues with slow overlays, please email firstname.lastname@example.org