When starting, the FPS will drop to 0 or 1 (even on superflat), and nearly impossible to play, but when pausing the game, it will show 15-25 FPS.

error:
.
[12:12:55 SEVERE]: Fatal error launching game. Report this to http://bugs.mojang.com please!
java.io.FileNotFoundException: File 'C:\Users\minec\AppData\Roaming\.minecraft\assets\indexes\legacy.json' does not exist

at org.apache.commons.io.FileUtils.openInputStream(FileUtils.java:299)

at org.apache.commons.io.FileUtils.readFileToString(FileUtils.java:1711)

at net.minecraft.launcher.Launcher.aE(Unknown Source)

at net.minecraft.launcher.Launcher.aK(Unknown Source)

at net.minecraft.launcher.game.a.B(Unknown Source)

at com.mojang.launcher.b.b.a.b(Unknown Source)

at com.mojang.launcher.updater.download.DownloadJob.popAndDownload(Unknown Source)

at com.mojang.launcher.updater.download.DownloadJob.access$0(Unknown Source)

at com.mojang.launcher.updater.download.DownloadJob$1.run(Unknown Source)

at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)

at java.util.concurrent.FutureTask.run(Unknown Source)

at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)

at java.lang.Thread.run(Unknown Source)