c++ std::async : faster on 4 cores compared to 8 cores

I have 16000 jobs to perform.

Each job is independent. There is no shared memory, no interprocess communication, no lock or mutex.

I am on ubuntu 16.06. c++11. Intel® Core™ i7-8550U CPU @ 1.80GHz × 8

I use std::async to split jobs between cores.

If I split the jobs into 8 (2000 per core), computation time is 145. If I split the jobs into 4 (4000 per core), computation time is 60.

Output after reduce is the same in both case.

If I monitor the CPU during computation (just using htop), things happen as expected (8 cores are used at 100% in first case, only 4 cores are used 100% in second case).

I am very confused why 4 cores would process much faster than 8.