If it goes too slow try playing in Turbo. FIRST: this project is made to save and load multiple users designed levels in the cloud. The game logic is buggy and unfinished because it's only for some testing purposes. At the beginning you will see a list of ONLINE stored levels, you choose a level and you can see and "play" it. when playing you must press "E" to go to the edit menu. (You MUST be "playing" a level to do that, sorry) To add blocks, press 1 to add a normal block, 2 for a lava block, 3 for the game start position and 4 for the game finish position. Also you can press 0 to choose the remover. Then just clic on the selected cell and the block will be added. Press "S" to save your level. Press "SPACE" to go to the initial select menu.
This is a very unfinished game, the part to save and load levels shoud work fine because it is the real target, the other one was just for some testing.