(MC Forge 1.18.2, Create 0.5.1c, C:DG 1.1d)

When an engine is active without a silencer, it rapidly plays a large amount of sounds at once (~40 per large engine, ~10 per small engine), stopping at around 200 concurrent sounds no matter how many are placed.

(Ignore ambient sounds)

On its own, this doesn't affect the framerate at all, though having this many sounds is still a problem. It may start preventing or cancelling out other sounds in a place that already has lots of audible concurrent sounds to begin with. This is the primary issue if you're not using Sound Physics or similar.

With a mod like Sound Physics Remastered, rapidly-spawned sounds like this (to this degree) can also severely impact the framerate. It gets worse the more unsilenced engines you power. The impact seems to kinda depend on how the sounds line up (build then power, build off powered engine, rejoin, etc).

In the above screenshots, I was using a modpack with ~250 mods, including Sound Physics Remastered and Dynamic Surroundings Resurrected (with the sound physics settings disabled). The above setup could drop the framerate anywhere between <30FPS and ~3FPS, depending on how the sounds line up.
I tested it in a more barebones modpack with only C:DG, Create, the sound mods, and a few essentials, and got basically the same issues. The framerate drops with sound physics weren't nearly as bad ("average" framerate was still in the hundreds), but it was still stuttering every few frames.

These issues could be solved by replacing the spammed steam engine sound with a new "engine audio loop" sound, particularly one that isn't so short. This would already be a nice QoL thing to have in the first place, but now it's also a significant optimization improvement.