Agree with piebob, you'll need to provide more details if you want any meaningful replies.
To answer your original query, we're up to 52 billion events without any trouble.
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