There has been a lot of changes made to the TBM between 1.1.4 to 1.1.8...
1.1.5 - Couldn't find the post 1.1.6 - not much in the update other than making synthetic vision a little less frame rate hungry. 1.1.7 - Seems to be more of a VR update and a under the hood for systems. 1.1.8 - Was a very small bug fix update so...
But like @DwightP980 said with out a little more info there's really no way to tell you. Things like full system specs, scenery an kind of scenery, plugins, anything new you've added and so on. Everything you add to XP has the potential to eat away at frames.
Have you tried removing all of your plugins and custom scenery and see if you get some frame rates back? Try deleting your shades and preference files and let XP rebuild them... Might help.
Not knowing your system specs but your AMD Radeon R9 M395X's low clock speed and VRAM cloud be playing a part in it to.