Video games are like using a firehose to wash your car.

Processing speed is a chemical/metabolic process. I don't think you can change it by pushing the envelope, at least not with a less than healthy brain. A healthy brain may be slow due to lack of focus and other issues. The injured brain is slow due to inefficient white matter. The health of the axons/white matter is the key factor in improving processing speed.

Stressing the brain with video games would be counter to supporting good health. A panic attack is an attempt by the brain to speed up processing speed but failing because the brain cannot handle the extra stress load.

Why do you think you have slowed processing speed ?