Most speeds were adjusted accordingly to make the game harder.
Lower levels were sped up 'cause they were too easy.
Level 9's speed was left alone so that the demo won't desync.
Levels whose speeds were adjusted: Levels 0 thru 8 and 15 thru 20
Levels 19 and 20 are now kill screens.
Visuals were taken from the NES version of Tetris.
Level 9❤ is virtually impossible for any human skill especially on B-Type.
Level 9❤ Heights 4 and 5 are impossible even with tool assistance.
Remember the kill screen that happens in level 29 on the NES version? It can now be experienced on the Game Boy.

Hardcore Tetris 1.0 must be patched to Tetris v1.0 (the Japanese version)
A-Type Level up every 20 lines regardless of level.

Hardcore Tetris 1.1 and higher must be patched to Tetris v1.1 (the Worldwide version)
A-Type Level up at ( 10 x level + 10 ) lines.
1.1 has the most memorable tune in Music A where 1.0 has a different tune.

Features:

1.0 and 1.1
Increased speeds, making level 19 and 9❤ virtually impossible.

1.2
Slightly reduced speed in levels 18 and 19, but are still hard.

1.3
Slightly reduced speed in levels 17, 19 and 20 which removes the kill screen

1.0 Classic Challenge and 1.4
Reverted speeds for levels 15 thru 20.

The current TAS of Tetris syncs run on versions 1.1 and higher.
The obsolete TAS of Tetris syncs only on 1.4.