Hello!
This could be something potentially fixed by an update I haven't released yet... (Enabling batching with the Ultra shader) If you'd like to try this version, please send me an email (with the address on my website) with your invoice no.! It would be a huge help, (especially if you happen to be building for android) but I really need to test this update more to make sure enabling batching won't break effect rendering before sending it out!
As a secondary option, try the Universal shader, add an "STMMaskableGraphic" component to your UI object, and then add an "Outline" component. This is Unity's standard outline method for UI objects and the STMMaskableGraphic component lets it work together with STM so I'd expect it to perform a bit better since it's much simpler. (I'm also due to attempt a clean-up of the Ultra shader again, I got very close to removing many unnecessary calculations, but there were one or two I couldn't find a way around and ran out of time I had so I had to revert it all)