There should no discernible performance difference pool vs. non-pool.
Here are the reasons why you might see degraded performance:
- The disk that the pool is reading from is different than the non-pooled disk that appears to be faster. So in fact, different disk are being compared.
- There is a 3rd party software, such as an Indexer that is accessing and indexing the pool.
- There is a 3rd party filter driver, such as an A/V that is intercepting and processing the I/O to the pool. (use fltmc on command line to check)
- File duplication is enabled so the performance will depend on which disks the file is stored.