There’s a lot of research showing that humans are able to deal with upwards of 2000 images with recall above 90% accuracy.
I’m not sure if it’s all humans, or if this has been tested with people who have aphantasia, but you can test it yourself during your Memory Palace creation.
For example, take a walk down any commercial street and go into all the stories, cafes, etc.
Then, draw their layouts from memory.
Finally, go back and compare your drawings to the layouts of the buildings. You’ll be astonished by your accuracy - even if you’re not a great artist.
When it comes to cards, you’re loading on more than visual information. You’re dealing with numbers, sometimes faces (if you operate that way) and abstractions like suit. Really, the only place suits exist is in a deck of cards, though it has conceptual parallels with hierarchies like a business or something like that.
This arrangement of information-types, which is at least 3 levels deep compared to just a picture, explains part of the difficulty with memorizing cards cold without a technique.
In my own limited competition experience, what slowed me down was switching between two techniques and relying heavily on the conceptual mode.
Since that time, I’ve sped up a fair amount, and that is because I’ve worked on making each and every concept more visual. But even recently during a demonstration, I still have the tendency to switch into the conceptual mode. I think that’s largely because it’s my favorite mode when learning a language or something language related, like my current Sanskrit project.
One book you might enjoy reading that goes deeper into this is called The Case for Mental Imagery by Stephen Kosslyn, et al.