To reproduce:
1. Open attached project "IL2CPPSocketPoll.zip"
2. Build for iOS
3. Deploy the Xcode project to the iOS device
4. Press "Receiver" button
5. Press "Poll" button
6. In Xcode open "Show the Debug navigator" and select CPU tab

Expected result: CPU usage is similar between 2017.4 and 2018.3 and later
Actual result: in 2018.3 and later versions CPU usage is 100%, on 2017.4 it's 20-30%

Reproduced on:
2018.3.0a8 2018.3.10f1 2019.1.0b8 2019.2.0a9

Not reproduced on:
2017.4.24f1 2018.3.0a7

Devices reproduced on:
-iPhone X iOS 12.1
-iPod Touch 6tg gen iOS 10.2.1 (CPU usage is jumping between 70-100%)

Notes:
-Getting the same results with Scripting Runtime Version .NET 3.5
-Doesn't reproduce on iPod Touch 6tg gen iOS 10.2.1 with Scripting Backend - MONO