Up to 10.14.6 versions of the latest Mac OS Mojave (and for some models even from Mac OS Sierra) have issues slowing down the HTTP calls when recording or playing back using the Test Designer engine.
The proper fix for this issue can be found in Apple Safari: Mac Configuration.
If you have already a Mac that was bought with Mojave or have already upgraded from High Sierra to Mojave, there is no easy way to downgrade back to High Sierra using the App Store, and depending on your model this might be even impossible.
So we would not recommend follow this path but better refer to the above document.