I think that is not because Gurner FX is too heavy but because the exhaust flame code there for Hurricanes, Spitfires and Mustangs creates a huge amount of flame effects to all exhausts of the engine at high throttle setting. All Merlin Hurricanes have six exhausts, so the code creates flame effects always to all those six exhaust effect hooks, and that works well for all Merlin Hurricanes. However, that flame code is also used for these Hercules Hurricanes since it is in parent classfile, but these have less exhausts and that causes the problem. Creating flame effects to the missing exhaust hooks causes errors in console and logfile, and there are a lot of those errors. Then the game stutters because generating that huge amount of error messages eats almost all CPU time.
However, I have to agree that Gurner FX is a pretty heavy effect mod in terms of effect texture sixe with all those animated textures. Thus, it may be too much for PCs with little RAM and graphics memory. Other problem is that dropping a nuke, cookie or some other large bomb into a city overloads the rendering engine with all those burning and smoking houses. As a result only some of the effects are rendered and you may not see the mushroom effect of the bomb itself at all, and all glass surfaces of planes will disappear when that destruction is visible. To fix that we could make only one tenth or so of buildings catch fire when destroyed and use some other short dust effect with a few particles for the others. That would be more realistic also since not every destroyed building catches fire and starts smoking, usually they just collapse with a cloud of dust.