Currently we are seeing a high influx of support request at Microsoft support services related to Sandboxes that were created as a copy of a production on which UAT stress testing is being done or planned. Currently it is not recommended to run stress tests on these Sandbox. Sandboxes are intended for development purposes, not for bench marking. You would never be able to compare the outcome of the test with a productive tenant.
Our technical recommendation is to run verification of solutions on production environments. Now you may create up to 3 Productions and 3 Sandboxes. Sandboxes are running on less performing hardware.
We are currently working on more documentation about Sandboxes and the purpose of it and will publish this asap to Docs.