very strange carp problem

[opnsenseuser2020]

i've came across a very strange bug.
scenario :
2 opnsense with ha cluster across 2 separate esxi hosts, everything works normal except the upload. the upload is always half of what it should be.
after a lot of investigation, it has something to do with the existence of a carp vip on that lan.

the problem : upload is always half of what it should.
the second i delete the carp vip, the upload goes back to normal. even if i'm not using it. even if i'm testing directly from the fw'ls IP. the very fact that the carp vip is there, slows down the upload to half, in any case.

i even setup 2 new opnsense firewall, and even didn't create a cluster, i just set them up for comparison. and there also, the second i add the carp for the lan, upload is half. the wan carp by the way, is not generating the problem.