MockService performance

Hey, i would like to ask if there is a way to improve rest mockService performance

My coworkers have task to perform end-to-end test with 20 threads simultaneously on system that uses 2 of my mockServices. The problem is that each acknowledged request has bigger response time reaching to 12 seconds in peak and then goes down to something like 200ms to repeat whole process. Down below i attached test results

I measured processing time of my code that is transforming request where it selects important data, add some generated informations to return as response and it does its job in 1ms. That suggests mockService request processing problems ... but i didn't figure out yet what is the reason. Request is not large, only 15 lines so there is not much to process ... is it everything because of 20 threads ? Can i assign more memory to perticular mockService or something ?