Errors, warnings and 10 second freeze on loading list of singleplayer worlds

When clicking "Singleplayer" in the main menu the game freezes for about 10 seconds and outputs various warnings and errors in the launcher log.

I have various old worlds that were last opened in various old versions in the saves folders and it seems that it fails loading the level.dat of some of them.

I don't think the level.dats of these worlds are corrupted though, because in 1.12.2 they load without any issues.

It seems there are two outputs that might relate to the freeze/fail of loading the level.dats.
14 times something like this:

[00:28:38] [Client thread/WARN]: Unsupported key: CommandBlock in TaggedChoiceType[id, MobSpawner -> MuType[V143_4], 
Chest -> ((Tag["Items", List[MuType[V143_3]]] | NilDrop), NilSave), 
Airportal -> NilSave, 
DLDetector -> NilSave, 
Banner -> NilSave, 
FlowerPot -> ((Tag["Item", (Int | NamedType["item_name", NamespacedString])] | NilDrop), NilSave), 
Trap -> ((Tag["Items", List[MuType[V143_3]]] | NilDrop), NilSave), 
EnchantTable -> NilSave, 
Furnace -> ((Tag["Items", List[MuType[V143_3]]] | NilDrop), NilSave), 
Dropper -> ((Tag["Items", List[MuType[V143_3]]] | NilDrop), NilSave), 
Comparator -> NilSave, 
Cauldron -> ((Tag["Items", List[MuType[V143_3]]] | NilDrop), NilSave), 
Control -> NilSave, 
EndGateway -> NilSave, 
Music -> NilSave, 
Piston -> NilSave, 
Beacon -> NilSave, 
Skull -> NilSave, 
Hopper -> ((Tag["Items", List[MuType[V143_3]]] | NilDrop), NilSave), 
Sign -> NilSave, 
Structure -> NilSave, 
EnderChest -> NilSave, 
RecordPlayer -> ((Tag["RecordItem", MuType[V143_3]] | NilDrop), NilSave)]

Likely relates to/overlaps with MC-130613.

And 10 times something like this (it fails for level.dat as well as for level.dat_old, so it fails for five worlds):

[00:28:39] [Client thread/ERROR]: Exception reading /home/panda/.minecraft/saves/Bunker_0.7/level.dat
com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 3 path $
	at com.google.gson.JsonParser.parse(JsonParser.java:65) ~[gson-2.8.0.jar:?]
	at com.google.gson.JsonParser.parse(JsonParser.java:45) ~[gson-2.8.0.jar:?]
	at cfa.<init>(SourceFile:108) ~[1.13-pre1.jar:?]
	at cez.a(SourceFile:94) ~[1.13-pre1.jar:?]
	at cez.c(SourceFile:76) ~[1.13-pre1.jar:?]
	at cew.b(SourceFile:67) ~[1.13-pre1.jar:?]
	at cph.g(SourceFile:35) ~[1.13-pre1.jar:?]
	at cph.<init>(SourceFile:27) ~[1.13-pre1.jar:?]
	at cpf.c(SourceFile:48) ~[1.13-pre1.jar:?]
	at cmh.a(SourceFile:311) ~[1.13-pre1.jar:?]
	at ciq.a(SourceFile:723) ~[1.13-pre1.jar:?]
	at cmm$4.a(SourceFile:212) ~[1.13-pre1.jar:?]
	at cjr.mouseClicked(SourceFile:103) ~[1.13-pre1.jar:?]
	at ckn.mouseClicked(SourceFile:45) ~[1.13-pre1.jar:?]
	at cmm.mouseClicked(SourceFile:352) ~[1.13-pre1.jar:?]
	at cis.b(SourceFile:67) ~[1.13-pre1.jar:?]
	at cmh.a(SourceFile:429) ~[1.13-pre1.jar:?]
	at cis.a(SourceFile:67) ~[1.13-pre1.jar:?]
	at org.lwjgl.glfw.GLFWMouseButtonCallbackI.callback(GLFWMouseButtonCallbackI.java:23) [lwjgl-glfw-3.1.2.jar:?]
	at org.lwjgl.system.JNI.invokeV(Native Method) ~[lwjgl-3.1.2.jar:?]
	at org.lwjgl.glfw.GLFW.glfwWaitEventsTimeout(GLFW.java:2599) [lwjgl-glfw-3.1.2.jar:?]
	at cja.a(SourceFile:298) [1.13-pre1.jar:?]
	at ciq.c(SourceFile:839) [1.13-pre1.jar:?]
	at ciq.a(SourceFile:379) [1.13-pre1.jar:?]
	at net.minecraft.client.main.Main.main(SourceFile:144) [1.13-pre1.jar:?]
Caused by: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 3 path $
	at com.google.gson.stream.JsonReader.syntaxError(JsonReader.java:1559) ~[gson-2.8.0.jar:?]
	at com.google.gson.stream.JsonReader.checkLenient(JsonReader.java:1401) ~[gson-2.8.0.jar:?]
	at com.google.gson.stream.JsonReader.doPeek(JsonReader.java:542) ~[gson-2.8.0.jar:?]
	at com.google.gson.stream.JsonReader.peek(JsonReader.java:425) ~[gson-2.8.0.jar:?]
	at com.google.gson.JsonParser.parse(JsonParser.java:60) ~[gson-2.8.0.jar:?]
	... 24 more