There's no difference between Laravel and Slim regarding the number of articles. In both cases Aimeos can handle in the default MySQL setup up to 100k articles. There's also almost no difference in speed between both (ca. 40ms reponse time for list and detail views in optimized setups).
The difference is how much you need to implment yourself. For example, Laravel already contains user accounts and registration, password forgotten and other things. You have to care about that in Slim yourself so for shops without user accounts, Slim fits well. For all others we would recommend Laravel.
How to configure and adapt Aimeos based shops as developer
Forum rules: Always add your Aimeos and PHP version as well as your environment (Linux/Mac/Win)