Depends. Sponge for the most part runs off forge so when forge updates to a new minecraft version with a stable release of forge, sponge wont be too long after it. This rule sadly is still in play even if you don’t go with the forge version of Sponge. Note that Sponge is not yet on 1.14 as the last fully stable version of Forge is for 1.12.2)
No. Compared to bog standard vanilla it actually boosts performance due to optimisations
Personally not tried it, but I would assume not much if anything.