Behaviour of performance counter "Checkpoint Pages/sec" under load
Checkpoints, like most things in SQL Server, are a big topic.  A good place to start is this Microsoft Docs page: Database Checkpoints (SQL Server)

To respond to your main question though, check out this specific quote from that page:


  The time interval between automatic checkpoints can be highly variable. A database with a substantial transaction workload will have more frequent checkpoints than a database used primarily for read-only operations. Under the simple recovery model, an automatic checkpoint is also queued if the log becomes 70 percent full.


So "checkpoint pages / sec" will vary based on a lot of factors:


the type of checkpoints you're using (automatic or indirect)
if using automatic checkpoints, the value of the "recovery interval" server-level setting
if using indirect checkpoints, the value of the TARGET_RECOVERY_TIME database-level setting
disk latency



  Automatic checkpoints are throttled based on the number of outstanding writes and whether the Database Engine detects an increase in write latency above 50 milliseconds.



the transactional throughput currently happening on the database


All that to say: seeing peaks and valleys in the number of pages being flushed to disk by checkpoint operations is completely normal.

If the purpose of the performance testing you mentioned is to normalize (as much as possible) the number of pages flushed to disk per second, consider reading the documentation page I linked to above and adjusting those settings.