why WEBrick is faster as dev server compared to thin as dev server ?

[metapraveen]

We are using thin in our gem file so our dev server uses thin which is started by bundle exec rails server

I observed that our dev server got slower after thin so I fired 50 parallel requests which loads the resource from db.
With thin slowest request took 58 seconds
without thin (WEBrick) the slowest request took 17 seconds

Why does thin slows down requests in dev server or am I missing some config here?
rails version - 5.2
thin version - 1.7

PS - thank you for this amazing gem. I am trying to understand how to use it make my dev server faster.