WAL restore is very slow

We have PG-14 with a huge data set of 14 TB running on r5b.2xlarge. We have set up WAL archiving and restoring them onto a replica server. The WAL restore on the replica is very slow and we are not able to achieve the 4 hour delayed replica. It is always behind 30 hrs with the huge WAL generation.

I have checked the following and they look fine

1. Bottlenecks on the replica server

2. Memory consumption and swap

3. EFS IO throughput

4.checkpoint_completion_target = 0.9

7. Verified logs and didn't find anything useful related to the issue

Can you please suggest how to improve the WAL restore performance 

Thank you 

Madhu Sudan