I noticed i was a bit offputting with my previous comment, and i didnt give a better (imo) analogy to help understand speed ingame, so here I go.
The TM bar is like a bucket that needs to be filled with water. Each character has his own bucket and his own faucet. How much water comes out of the faucet (per minute) is determined by their individual speed stat. When the bucket is full, the character needs to empty it (take his turn) and start over with filling the bucket with water (at the rate determined by his speed).
The process of filling al 10 buckets with water is sped up until a character has a full bucket. The main valve is now shut until the character has emptied his bucket (took his turn). Once this is done the main valve will be opened agian, and the process of filling the buckets is sped up until the next bucket is full. etc. etc.
You could imagine +50% TM as a half bucket of water emptied into the character's bucket that recieves the boost. Every drop of water that spills over is wasted. Same goes for -50% TM, only now the water is taken out of the bucket. Cant take more water out of the bucket than there is water in the bucket, so if the character's bucket is filled for less than 50% the TM reduction is just wasted.
so basically all the characters percieve time the same way, but faster characters just have a bigger faucet and fill their bucket faster. For our (gamer/user) convenience the process of filling these buckets is sped up, and paused (main valve shut) when characters take their turn (empty their bucket)
edit: granted, this is a lot less interesting to read ;)
