The issue isn't with FSUIPC, it's with anything that uses the 64bit MS/Asobo SimConnect implementation heavily. There has been a regression bug since the release candidate that was provided for testing pre-release.
It is highly unlikely that MS/Asobo intends to steal defeat from the jaws of victory by making MSFS2020 useless to serious simmers and pit builders. The in-game Marketplace is for DLC-type content -- aircraft, airports, scenery. None of that is diminished by the apps and extensions that rely on SimConnect.
If MS/Asobo didn't want to provide that functionality, it wouldn't be there at all -- rather than in a bugged form that renders the product entirely unusable.
As suggested upthread, given that the release candidate build didn't have this problem, it's very likely that the "bug" is that the shipped version was mistakenly compiled with some kind of simconnect debugging enabled, resulting in wait states that make the sim slow to a crawl if constantly polled for data by something using FSUIPC, which in turn requests data via the 64bit SimConnect API (in my case, OnAir's client).
The wait is frustrating, but there's approximately zero chance MS wants to cede this ground to X-Plane.
