Can low PCs connect to servers with no issues?

So I’ve just got an i9 9900k with 32 GB of ram. If I were to run a modded server, could my family connect to it with lower end PCs with little to no lag or issues?

Is there anything I can do to make them able to join issue free, obviously other than get them better computers?

How and what affects lag and FPS drops? I’ve read that’s the main issue with Minecraft. Can we use it with optifine? Should optfine be installed on server?