I'll try and keep my tone out of this post, but just a heads up that upgrading to version 12 (on prem) has caused nothing but headaches for me and my team. 

We did an upgrade from 11 to 12 about 2 months ago on this machine. Immediately users could not add more than 6 rows to a collection without the browser showing a pop up 'Wait' window. It's more than just that though, users have to wait over a minute for a new collection row to show. 

This is on classic designer with a ton of jQuery. I decided to 'dissect' the form to see what made it tick. I removed all jQuery and lookups. The problem remained. I removed all field rules, although they added a small amount of speed, the form is still useless. The culprit was this formula in the collection: =ROW(). Meanwhile I have a huge formula that defines a full month name based off a number. 

As soon as I removed the =ROW() formula, it picked up some speed. The only other way to gain speed is to remove field rules and replace them with jQuery. That became exponentially complex. I could get it up 20 rows before it slows down. These users regularly enter 30 to 50 rows into the collection, so this is a workaround. We have the users submit blocks of 20 at a time and they utterly and completely hate that. Also they hate my team now, so that is awesome. Again, this form has been in use for years. Now we have a crisis when users go in to get reimbursed for travel. 

I converted the offending form to modern designer. The problems remains. Surprisingly classic designer is faster by a noticeable margin.

Does any of this sound familiar to anyone? If so how did you handle this? I have another process that has slowed down after upgrading to 12. My VAR tells me their experience has been the exact opposite, so I am confused. 

Thanks friends!