Does having a lot of text messages and call logs decrease the speed of an iPhone?

Does having a lot of text messages and call logs decrease the speed of an iPhone?

No, since the iPhone has flash storage which does not slow down as the storage is filled.

Trust me, the amount of space you have on that device can increase or decrease the speed of the device along with how much RAM you have available and which apps you have running in the background. I would know since when I free up space on my phone it becomes faster.