Free List Stall per Second: 

Its a counter available in SQL Buffer Manager. If SQL Server don’t have free pages in buffer cache then a processing request goes in stall and it must wait until pages get free in buffer cache.

The recommended value of Free List per Seconds counter is below 2.

If there is Free List Stall per Second counter’s value is above 2, then check Page Life Expectancy Value and if you found it’s less than 300 Seconds then also verify Lazy Writer per Second value. If Lazy Writer per Second having value higher than 20, then it indicates that the system is running in memory pressure.