cpu issue on when using less

[凯夏]

When I use less read from pipe,even i just output a empty new line to less,
the bash running less gets high cpu usage ( picture 1, picture 2 ), and
less itself didn't use many cpu, when i quit less, cpu usage of bash drop
to normal ( picture 3 )
When I use less read from file,the bash running less gets low cpu usage
How can I debug this?
I post these three snapshot on imgur if you can't see them:
https://imgur.com/a/mDfH8Dl