I run a very large number of web performance tests and GA and the Facebook pixel do not add 400ms either alone or together of user-perceptible speed decreases. They will take some small amount of main thread time for parse and compile, but they are loaded async and generally not performance problems on the (hundreds) of pages I have profiled.

Intercom and other live chat solutions are typically the biggest offenders on modern pages. They serve 10-20x the script as GA and the Facebook pixel.