Ajax send request per second and make hight cpu usage

I have application which send request per second to view and there I get data from api and return response from client so

• Ajax(send request)->
• View(Send request)->
• Api(Return response for view)->
• View(Return response for client)

I saw that's uses a lot of CPU.

1. Why it is usages a lot of cpu?

2. How I can optimization it? - I must press point that I download currencies and I need download data per second

3. Should I use WebSocket?