Wow...that's very interesting.
I've had the opposite experience so far....
I was actually getting ready to post an update here about the 2019 version. One thing it seems to have addressed on my old computer, is it seems significantly sped up.
I can now use my wacom tablet and pen with it and it moves smooth without the lagging that I got from the 2018 versions.
Also most other uses seems smooth and quick and fluid, compared to my experience at times with the 2018.
What computer and OS are you using?
I have an old late 2011 MacBook Pro. I've maxed it out with 16GB RAM. I upgraded the internal drive with a samsung SSD, and I work most of my images stored on an external thunderbolt to SATA spinning disk, and have a thunderbolt to SATA unit holding a SSD for my cache.
Also, I downloaded the 2019 version, not realizing that my version of OSX (Yosimite) was not going to work and I had to do an upgrade to High Sierra in order to get On1 RAW 2019 to install.
As far as drawbacks....
I found that the focus stacking is till a bit wanting....I didn't like the images at all on the On1 render and ended up sending my exposure to PS (my old version of CS6) to focus stack for a project I was working on.
I have found one complaint...that the cataloging search function isn't working for me as I expected. I have all my shots for the year in a 2018 directory on my external drive. I have this as a cataloged folder.
I have gone through my Jazzfest shots from last year and marked them in blue label as my 'hero' shots.
I click the 2018 folder, and in filters (browse mode), I select the blue label...and NOTHING shows up. I've opened a support ticket with them.
But goodness, I've not experienced anything like you have!!!
One funny thing about my install. I actually installed and told it to UN-install my 2018 version....but apparently it did not uninstall it, as the the other day in finder I typed in ON1 and I didn't notice the auto-complete had chosen 2018 instead of 2019.
So far, I've found them to be pretty good in quick customer service and response. Please keep us up to date on what happens here with you.
Again, curious, what type of hardware and OS are you running?
Thanks in advance,
C