Here's a thread on Image-Line for more info: https://forum.image-line.com/viewtopic.php?t=193230
And here's a video showing Serum knob turning in FL Studio v.s. Ableton: https://forum.image-line.com/download/file.php?id=194280
Serum has noticeable visual lag in FL Studio (Windows), despite it being the only VST in my project and my PC being spec'd out (1080 TI + i7-8700k). The ^ thread shows that this doesn't happen in Ableton. Also this lag doesn't exist in your OTT plugin when I use that in my project. Another thing is you can notice the low FPS when moving around the Envelope curve (30 FPS) v.s. the LFO curve (60 FPS). The only difference between moving the two curves is that when moving around the Envelope curve it also turns a couple of knobs (ADSR). So maybe its only the knob turning that's causing the framerate drop, and it may be tied to how FL Studio is constantly keeping track of knob amounts (e.g. it shows the knob amount in the hint bar in the menu bar).
If there's any advice for fixing this I would greatly appreciate it! And if it's a bug, I suggest looking at OTT's code when adjusting knob amount and seeing how it differs from Serum's knobs.