We've been also having big performance issues on 7.8.1. Cluster was set up from scratch on Kubernetes with official Helm chart. We imported the same data we have in ES 6.8 and even with doubled resources we are not able to handle same amount of load as before.
Performance degradation is mainly manifested with heavily slowed down indexing times (rest bulk requests are taking up to 30 seconds vs standard few ms). But response time of http we are seeing from our web app is not fully reflected in increases indexing times shown in Stack Monitoring though.
Our 6.8 cluster has 3 master and 6 data nodes (16GB ram, 10GB heap, 4 CPUs, 300M documents, 36 indices, 150 shards, heavy indexing)
EDIT: We are also seeing increased CPU usage (2x-4x) with 7.8.1 like described in High OS Cpu usage on 7.7.1