Pictures in the webinterface trigger the generation of preview images which can slow down a lot of things. To see the impact, you should see the difference of a folder that you have visited before compared to a new folder with freshly uploaded pictures. To help with that, previews can be pre-generated by cronjobs (the nextcloudpi-project does such tricks to speed up the performance of the Raspberry pi).
RAM can make a difference if used correctly. There are the caches (redis, apcu) for php, and you can also optimize the caches of your database. This can make a huge difference in the data transfer speed.
From NC21, there is the push notification high speed backend (Setting up Files (High Performance Backend) - #6 by nursoda).
That certainly isn’t limited by your disk. If you have decent amount of RAM and the caches set up correctly, the network connection is the limit rather than disk speed.
For the processor, I honestly don’t know. After optimizations, you can check the CPU usage a bit for that.