The current dimension stack feature advertises that it degrades performance. A better implementation, which completely replaces the current one, follows:

"Holes" in the bedrock layer spawn like structures/features with configurable frequency. This is a 4x4 frame of bedrock with a horizontal immersive portal in the 2x2 center. These spawn in the bottom layer of the bedrock and do not prevent other bedrock blocks from spawning overtop of them.

Further ideas:

• If a spot chosen by the portal is completely covered by bedrock in the upper layers then the portal fails to spawn.
• A configuration option exists to disable vanilla portals using obsidian frames.
• Rather than spawning active portals, the implemented structures/features carve spots in the bedrock which are particularly suitable for mod-added "digging devices" which create the portal.

The current version already allows disabling nether portal functionality by seting nether portal mode to disabled.

Optimizing dimension stack performance by not loading the other side if the bedrock is intact is a good idea.