Doing speed and strength training together on the same day will not optimize your speed and power and make you slower. However, there are strategic ways to combine them. But first, understand the process.
If you want to get faster, you need to reduce the weight you are moving. As resistance and force goes up, speed goes down. Therefore, speed requires a reduction in resistance in order to move fast. You can simply see this by doing a 1-rep max in comparison to an 8-rep max. Just training strength and heavy all the time, will slow you down. Strength training, lifting heavy is good. It has a place and time. Lifting heavy, using 5 reps, for a 4-week training cycle is perfect start before implementing speed training because it increases the rate of force. However, there has to be a crossover, a transfer.