What Bluetooth stylus are you using?
I use a Adonit Pixel with my 2015 iPad Pro and it is still working great for me in 2018!
My Adonit Pixel stylus is no longer “officially” supported on Procreate, either, but I contacted the developers of Procreate about the newer updates and they informed me that the previous version compatible stylus drivers are still installed should still work as they did before and “lo and behold” they were right and my Adonit Pixel stylus is still working well with Procreate, both pressure and palm rejection are still working.
Tayasui Sketches updated their stylus drivers a few updates, ago, also, and that app is working well for me, also.
ArtRage and Tayasui Sketches and Concepts drawing apps are my favorite and all STILL support my nearly two year old stylus
And Procreate and AutoDesk Sketchbook are still working with my two year old stylus, too!
Another thing I now use to draw is one of those non-electrical conducting lycra drawing gloves.
This helps with palm rejection issues a lot and if you use this type of glove, you can use ANY stylus with any drawing or notes app that doesn't support other smart Bluetooth stylii and do not have any palm rejection features.
The drawing glove allows you to rest you hand/palm on the screen while you draw or write and it won't leave any stray markings in these apps.
Some performance tips and advice to help your 1st gen iPad Air out.
Have you tried a hard reset of your iPad by holding down both the Home and sleep/wake buttons until your iPad goes to black and restarts with Apple logo, then release the buttons?
How much free data storage space is left on your iPad?
iDevices need to maintain, at least, 3 GBs, OR GREATER, of free data storage space.
If you need to free up free data storage space on an iPad/iDevice, you need to start by backing up your iPad to either Apple's iCloud or by connecting your iDevice to a computer running the latest iTunes.
Then delete any installed apps that you no longer use or use infrequently enough that they aren't missed (you can always reinstalled any purchased or free apps at some time later, as you need them).
Thin out/delete any music/videos/movies/TV shows/images/eBooks/eMagazine/iBooks/audio books/Documents/PDFs, etc.
Delete any old saved emails that maybe saved to your iPad.
To permanently delete photos/images off of your iDevice go into the Photos app and find the Recently Deleted folder/album and permanently delete any photos/images stored in that folder/album.
Also, you can check to see what stuff is eating up all your valuable data storage space by going into the Settings App,,General Settings Panel, in the right hand column, tap Storage & iCloud usage panel, in the top Storage Panel, tap the Manage Storage panel, wait for a list of apps to generate that will tell you how much space each app is taking up on your iDevice.
In Settings app, General settings panel, turn OFF Background App Refresh for any apps you feel do not need constant data updating/Internet updating in the background when not using the an app OR turn OFF Background App Refresh entirely/globally for the entire device.
In Settings app, under General settings, Reset panel, at the right bottom of the list, Reset All Settings.
In Settings app (NOT from the iOS Control Panel) turn OFF Bluetooth when not using any Bluetooth devices.
Also, in general, if you want a faster IDevice, on all of my iDevices, I turn off most of the iOS motion graphics eye candy, by simply turning ON Reduce Motion in Settings app, General, Accessibilty settings.
Make sure you aren't running scads and scads of background apps in the iOS 11 Control Panel/App Switcher.
If you are, you need to quit the bulk of these background running apps by tap and hold a finger on an app window in the switcher and slide your finger upwards to quit an app. You should be able to use more than one finger to quit more than one app window simultaneously.
Also, make sure you don’t have scads and scads of active website tabs running in the web browser.
If you do, greatly reduce the amount of active website tabs your web browser.
Some minority of user have been turning ON and leaving ON iCloud backups continuously backing up on a constant and daily basis.
If you are one of these users, turn off iCloud backup and only turn this on to do daily backups either when you are not using your iPad for a prolonged period of time (like when you are sleeping), then turn iCloud backups OFF when actually using your iPad
OR
only perform your backups on a lesser time schedule.( like twice a week, once a week, once every two weeks, once a month, etc.)
Tap on the link below to my iDevice/iOS performance/optimisation user tips link.
https://discussions.apple.com/docs/DOC-10344
I mention some of my tips in this post, already, but there are many other things to try, as well.
Try all of these tips and leave each tip active together. Do not simply try every tip individually, expecting just one tip to be a performance “cure all”.
Many of these tips are designed to work together to improve overall iOS/iDevice performance.
Good Luck to you!