When I first got my superhub 3 I use to get speeds 150mbps+ but now I can't even keep a stable speed between 2mbps to 20mbps (mostly 2-6). I understand that stable speeds aren't guaranteed but I'm confused as to how I can't even come close to the speeds I once could. It's pretty frustrating ngl.
You mention wifi speeds - have you got a device connected by ethernet cable to the Hub (eg a PC or Mac) to do a speed test and rule out a cable performance problem? And you've checked all the power leads and cables to see they are properly connected, pushed home, and undamaged?
If you've ruled that out, then you should be getting much better over wifi, around 40-80 Mbps for a 2.4 GHz connection, and 100-200 Mbps over 5 GHz.
Can you connect a device to 2.4 and then to 5 GHz, and tell us what speeds it achieves over each?
If the finger points to the wifi rather than the cable connection, it could be a fault with the Hub, but I'm more inclined to suspect that there's some local change to the wireless environment, for example wifi signal interference, either from competing routers, or certain devices like poor quality LED lights, improperly unsuppressed fluorescents, baby monitors etc. When interference occurs it is usually pronounced on 2.4 GHz connections, but comparatively rare on 5 GHz.
If you download the Android Wifi Analyzer app, that will show any competing wifi signals (but read and understand the pink-sticky posts in the wifi section of the forum before changing anything), but it won't show non-wifi interference. For that you'd need to establish if the problem is intermittent and only occurs at certain times.