If you use iMessages a lot then you know that the performance can become sluggish over time. The reason for this slowness is because messages keeps a history of contacts you have messaged in a group or individually. This list overtime becomes big and unwieldy. Messages will could then become slow at populating when trying to start, find or contribute to a chat.
This great tip from The Sweet Setup, illustrates how to clean up that list and speed up the performance of the Messages app. To begin, start a new message. Stop typing after entering the first letter of the name you want to find. Then select the information icon next to that contact or group and finally select to ‘Remove from Contacts’ in the next screen.
WARNING: Only remove entries that you do not want in your contacts any longer. Essentially, people or businesses you have no desire to chat with again. I find this especially helpful with the group messages and the ‘contact’ entry iOS creates for them.