this game is actually a pretty easy speedrun, its just about remembering and knowing where to put ur cursor. despite having the w/r (at the time of creation) i made a lot of mistakes in my run and i will probably correct them at somepoint, anyways on to the guide.
so the first part of speedrunning this game is the understanding of the dialogue boxes. something you'll realize if you try to imitate my run is the speed that im clearing the dialogue boxes, this is from a bug which is a bit hard to explain but essentially you have to use the menu screen and over over exit to main menu, you'll hear a ticking noise and if you just hold down the button at a random time in the ticking noise you'll get a random speed to your left mouse inputs, this allows you to hold down the mouse button and just speed through dialogue boxes.
once you have the highest tick you can get (unknown to me) you'll want to start a tutorial level game and make sure that the dialogue box is open, then press the menu button and leave so that your dialogue box is still up on the main menu, this allows you to know exactly where to aim when you get in to the level.
now on to the actual run, the first and most important thing is ALWAYS PRIORITIZE DIALOGUE BOXES. they are the only time constraint in this run out of your control, do all the actions you can in the time you have waiting for the next right arrow to light up. you have a lot of time to wait for them so dont worry about rushing too much especially at the start of the run. something else to note about the dialogue boxes is that if it tells you to click the play button, wait till the box disappears before you click stop other wise you will have to wait again for it to disappear (SOMETHING TO NOTE HERE im pretty sure a lot of the dialogue takes the same amount of time if you dont click stop as if you do but i just do it so i dont need to waste time/i need to move a piece.)
you should be able to easily click through the first dialogue and then click the play button as fast as you can, this just takes practice and if you have played games like osu! it shouldnt be hard.
after you have clicked stop dialogue box 10 and 11 will come up, click through these and then begin moving your barrel and plank up to prepare for dialogue 23 and 27.
next part is a pretty neat trick i saw in rias' run which im sure you can optimize down to make it a 11 through 23 skip instead of just 18. the key to this trick is just making sure youre looking down at the floor and then skip through dialogue when it comes up then look back up, not very hard but i think its quite cute.
while 18 is waiting for the right arrow, i either get ready for 23 and move the barrel a bit or i move the plank a bit closer, but be careful because if you need to adjust the barrel when 23 comes the plank can block it, so dont move it in position yet. once its in position i normally spam click, but there is probably some timing to it. do the same stuff for 27.
next you'll get to 29 which is really easy, click play and wait for the dialogue box to disappear then pause and click through to 34.
at 34 move the rat up to the position highlighted in green and move the dialogue box to 39, then spam click it.
click through to 41 then move the batter to about underneath the wall above the ramp, this normally gives you good results as i had many times where i didnt correctly place the batter.
then click play and get ready to hit time, as you spam through the next pieces of dialogue you'll near the end which is when the last box of dialogue comes up, you wont be able to read it because you're spamming but it says something like "now enjoy playing bad rats" idk im not too sure.
thats essentially it, wall of text aside this is the basic guide to speedrunning bad rats with the current understanding that i have. hf and feel free to pm me if you have anything interesting to add or good questions and ill be sure to add them in or answer.