To Reproduce Steps to reproduce the behavior:
Customer tries to create an instance in RHOSO environment where Nova API is relatively slow (takes ~1.5 second to process /v2.1/flavors/FLAVOR_ID/os-extra_specs request). Although Horizon is not causing this directly, requesting extra specs sequentially for list of flavors is clearly suboptimal.

Expected behavior
Horizon requests extra specs, or any other entities based on existing list in parallel.

Bug impact
When Nova API endpoint is slow, sequential approach causing very significant delays that could easily be avoided.