just tried it with turned off Virtual Radar. Seems to be fixing this issue with iOS12 on an iPad Air 2

We have reproduced the lagging issue on iOS 12. After thoroughly profiling it, we've so far determined the problem is deep within the graphical drawing routines of iOS. As for why it gets worse over time, we have no idea.

My main job today is to perform further instrumentation to determine if there's any way we can temporarily work around it by changing the manner in which we perform certain drawing, however this appears to be a problem with the operating system so it's unclear whether I will be successful and even if I am, it's likely to only be a temporary fix. I will report back tomorrow.

Although, generally its been performing better than my newer iPad mini 4 (iOS 12.0 with SD 3.10.4) did the weekend before, I have been experiencing freezing and restarts - even on the older iOS. I suspect the restarts / freezing on my older iPad could be due to battery age as iOS 11 introduced CPU throttling when the battery underperforms.

Anyway, I'm now stuck between two evils. I really want to use my newer iPad mini 4 with the faster cpu, but I haven't been able to successfully downgrade to iOS 11.4.1. I have the ipsw file for it but the option key / restore using iTunes on my mac isn't bringing up the file browser. Its always forcing an install of the latest iOS. I've done countless google searches and I understand the instructions, but iTunes isn't playing ball. iTunes/iCloud backups are meaningless, as they don't backup the firmware so be warned.

UPDATE: Re the iTunes restore, I've successfully downgraded to 11.4.1. Turns out the option key isn't recognised over screen sharing (I use a mac mini for all backups and remotely connect to it via my MacBook). Phew!!