Ah okay, if performance is of concern then trying to uninstall “everything that is not needed” is a valid motivation, too 
That being said, the SimConnect DLL does not poll by itself. It is merely the “messenger” between FS 2020 and whatever 3rd-party add-on (be it an add-on that is installed as an “extension” in the community folder, or a “stand-alone application” such as Little Navmap).
So yes, it is my understanding that even (complex) “extension” add-ons - even though they are “technically running within the context of FS 2020 itself” - may still use SimConnect to communicate with FS 2020. Note that I am not an add-on (as in “extension of some flight model” etc.) developer myself, so I am only citing what I read in the Software Development Kit (SDK) documentation.
(But yes, I am currently developing a stand-alone application myself, using said SimConnect.dll).
Again, SimConnect.dll by itself does not do any work on its own; there must always be an actual “client software” (add-on in the community folder, an external application, …) that initiates the connection and then subsequently asks for and sets “simulation variables” (such as aircraft altitude, pitch, bank, latitude, longitude, …).
Or in other words: simply uninstalling that “SimConnect Client” package won’t give you any additional performace by itself; however you are likely to find that some add-on (or possibly external application - whoever installed that “SimConnect Client” package in the first place…) stops working. And that might give you then some additional performance (once you also uninstall / stop using that add-on)
Personally I only have some sceneries and liveries installed (naturally they do not “interact” with FS 2020, they just modify / add (terrain) data), plus the FlyByWire A320 mod (version 0.5 something or so, the one which works with the latest FS 2020 update). But even that mod did not install any such “SimConnect Client” package (it interacts with FS 2020 via the “normal extension points” for add-ons).
Maybe you have some additional aircraft installed which might have installed that “SimConnect Client” package (some “legacy aircraft” possibly that were ported in a “quick and dirty” manner to the new FS 2020, and hence (still) communicate via SimConnect.dll instead of whatever is now provided by the FS 2020 SDK?). Who knows… but you’ll find out soon enough
But I can assure you that neither the FS 2020 (at least the “Steam” version) nor (not even) the “MSFS SDK” have installed that “SimConnect Client” package - that must be coming from some add-on / external application that you once installed…