One thing you can do is run it in steps for both the compaction and the free so you have performance effect data on your actual system. Do a run to compact and free of maybe 1 or 10 Gb.

Be ready for the WIJ to expand.

Try it on a non production copy if you can first and run an Integchk on the dataset after.

Make sure you are on a current version of Cache or IRIS since some versions had issues.

Know when your system really has it low utilization period. You might have midnight/end of day processing or someone might do the weekly ETL on the weekend.

Intersystems has worked hard but encryption is not free. Do you have something like batch processing? You would notice an impact more there then on a 1 second query. I would worry more about key security and recovery. Encryption also has a serious impact on the ability of modern SAN storage to dedup and compress which could result in higher than expected storage costs.