I'll concur with piebob and erga00. I also have over 10B events indexed without issue.
If you have a single server on older hardware with default network setting, you might be seeing issues during large searches.
Things that will be helpful in understanding why you might be seeing slowness;
number of Splunk servers:
If more than 1 server, what roles do they operate:
server hardware (memory, CPU, NIC (speed and #)):
splunk LFCs or UFCs reporting to indexers:
index=* host=* earliest=-7d | table host | dedup host
number of searches run (Expensive searches):
Search > Search Activity > Search details