We upgraded the Splunk Universal Forwarders on our web servers from 8.0.5 to 9.0.1 back in late October and since then we've seen a dramatic increase in CPU Utilization by the Splunkd.exe process on each server.
Each instance is tracking a fairly large amount of files - typically 3k or so per day and in a folder that can contain up to 10k files. I've found reducing the amount of 'old' files in the folder helps, but the CPU load is still dramatically above what it was with version 8.0.5.