This is almost certainly not because of the RAM File System speed, but because your Edge ends up with no cache, so everything is fetched from the network, which is much slower.
A RAM volume starts empty at every boot, so if the folder your symlink points to doesn't exist on it, Edge quietly stops caching.
To check: Restart, exit Edge fully (it often runs in the background), and paste your symlink path into Windows Explorer address bar. If it isn't available - that's the cause.
To fix: In the RAM File System window, select the volume where your Edge cache should be, choose
Edit - Properties, and:
- tick "Create automatically at startup",
- under "Create Folders", enter the folder name your link points to. For example, enter 'EdgeCache' if your link points to 'R:\EdgeCache'. The listed folders are recreated at every mount, so the link will then always have a valid target.