Respawning the Ender Dragon?

I recently decided to try and get back into minecraft after years of absence so I loaded up an old survival world of mine and started playing again.

I decided that I wanted to fight the Ender Dragon a couple of times for xp so I got my gear crystals ready and went in the End.

Tried spawning him twice. The crystals on top of the towers got placed but no Dragon. Closed and reopened my game, spawned him a third time and it worked but my game now runs at 5 fps.

I already rebooted my game and pc multiple times and it hasn’t fixed it. I’m playing Java version of Minecraft.

Anyone knows why this might have happened?