The CPU load will drop by quite a bit. It won't go very far down, but depending on builds in your current world, some 30-50% drop in load is to be expected. There will still be network overhead and don't expect your elytra performance to improve drastically (lots of data - new chunks - to be transferred to your client, unpacked and rendered).
You should also be aware of some significant differences between single-player and server play; in particular you can no longer pause the game, and spawn chunks remain loaded even if you're in other dimensions.
Before you go that way though, you may want to look into lag-mitigation activities, that may restore your 'integrated server' to a better shape. In particular, migrating laggy stuff out of spawn chunks (or migrating the spawn chunks away, using /setworldspawn in a lag-friendly rarely-visited area like a deep ocean), getting rid of big clumps of animals (e.g. "cow hole" type cow farms - where collisions are calculated between each possible pair, so 24 cows generate 552 collisions each tick), make sure all farms can be (and are) switched off when not in use (and preferably distant enough they are unloaded entirely), reduce the number of hopper minecarts with use, cover hoppers that don't need to suck in items from the world (hopper-lines) with composters,
And before anything else, install lag-reducing mods - Lithium, Phosphor, Sodium (and Fabric mod loader) - they don't break even very obscure vanilla features (a.k.a. useful bugs) while improving performance significantly; Lithium and Phosphor are server-side mods (but benefit the single-player integrated server too); Sodium is client-side, and mostly affecting GPU but it does perform some CPU optimizations as well.