When indexes are built from an application or a mongorestore there's no way to see if it's ongoing in the UI.

There should be an indication in the "Real-time" tab saying that an index on collection X is in progress. This would explain why performance is currently impacted.

It would be good to see index start and end marked on the "Metrics" visualizations so we can see the impact of index builds on IOPS, CPU, memory etc. This could help understand if we need to upscale the cluster.