FPS fluctuations are entirely due to the client computer's hardware and software, not the server, so Opis won't help there. Opis is good for finding server lag which manifests as block lag or slow GUIs—though those can also be caused by network congestion at either end.
Slow FPS will usually be due to the graphics card being asked to draw more than it can handle or the CPU working too hard between those render draws and falling behind. In the past, I have downloaded a server world folder and run it locally in creative to find the source of the problem. I started deleting things one-by-one from the affected area, constantly pausing to measure the frame rate at different locations and directions. In that case, there was no one thing causing the drop, it was just... too much stuff. When most of the stuff was gone, so was most of the slowdown.
Check the specs on your graphics card and find a site that rates it against other common cards. If it rates poorly, that could be the bottleneck in your system. Whether it is or not, the slowdown is probably due to all the stuff in your base. Minecraft is not super optimized and only single-threaded (so multi-core CPUs don't help much) so I find that getting 30 FPS in a heavily built-up base on a modded server is actually pretty good...
