Just throwing my hat in the ring on this problem. I have also experienced this recently. Vertical scroll works as expected but horizontal scroll is ridiculously slow. I can't offer any real solutions but have listed my observations below. The slow scroll is a workflow killer.
I am experiencing this on 2 different Mac systems. 2015 Macbook Pro (my home system) and a 2014 iMac (My collaborator's system at his house). Both systems are running Logic 10.4.4 on Mojave. My mouse is a Logitech M305. I use the same mouse on both systems. I have the Logitech Unifying software installed on both so that I can pair the mouse to each systems as I move between them. I also have a Logitech K360 keyboard that I move and pair on the same systems along with the mouse. The 2014 system works correctly with the Apple Magic mouse but I can't use that mouse as I find it to be an ergonomic nightmare for my hand and wrist.
I seem to recall this problem starting after I upgraded both systems to Mojave 1-2 months ago. Prior to that upgrade I had no issues with the horizontal scroll. Because the Logic version had not changed, I attributed this to some change Apple made in the OS. I couldn't find any OS settings that seemed to improve the horizontal action without also affecting the vertical action. I tried uninstalling/re-installing the Logitech drivers and software, but again, no improvement, so I gave up and resigned myself to dealing with it.
An odd thing has happened within the last 2 weeks. My mouse now works as expected on the 2015 Macbook Pro. The slow horizontal scroll continues on the 2014 iMac. I thought maybe the recent security update I did on the 2015 system was the reason for the improvement but that update resulted in no change on the 2014 system. The solution remains a head scratcher.
I'm glad to have proper mouse function back on my 2015 system because I do most of my work there. If I manage to get the 2014 system working and actually know why I will report back here with a solution.