The multipart library you're including is causing lag:

https://github.com/miyo6032/MultipartEntities/blob/master/src/main/java/io/github/stuff_stuffs/multipart_entities/mixin/common/MixinBox.java

This mixin causes a simple math operation to become expensive due to injecting into it which creates a new callback object on each invokation

can you send me the spark link - or how you got it to have the lag, can't produce it with a minimal setup