How many files are you monitoring in that directory? Is it a sub directory of a parent directory being monitored?
If there are lots (hundreds/thousands) of files in the directory, then Splunk has to cycle through all of them to detect change. If it makes sense, applying a whitelist for more targeted monitoring may help with speeding change detection. Or configure a single input per file/source in the directory instead of monitoring the entire directory.
Additionally, a more drastic approach is to consider increasing the number of file descriptors Splunk uses for monitoring inputs. The default is 32 FDs, which means Splunk uses a sliding window of 32 files to check for change at any given time. Try increasing (doubling or tripling) this to see if it helps. But you should try the other options first.
This parameter is controlled in limits.conf:
[inputproc]
max_fd = <integer>
* Maximum number of file descriptors that Splunk can use in the Select Processor.
* The maximum value honored is half the current number of allowed file descriptors per process. (ulimit -n /setrlimit NOFILES)
* If a value chosen is higher than the maximum allowed value, the maximum value is used instead.
* Defaults to 32.
