To reproduce:

1.Download and open attached "New Unity Project.zip" project.
2.Go to player settings and make sure Multithreaded Rendering is turned off.
3.Build the project on a device.
4.Focus On Input field.
5.Unfocus from Input field by touching anywhere on the screen.
6.Repeat steps 4 and 5 from 5 to 10 times.

Expected result: Camera does not lag when focusing and unfocusing on Input field couple of times.
Actual result: Camera starts lagging when focusing and unfocusing on Input field couple of times.

Reproduced with : 2017.0.1b1, 5.6.0f3, 5.5.3p2

DUT:

Verizon SM G930V (Galaxy S7 Verizon), OS:7.0, CPU:arm64-v8a, GPU:Adreno (TM) 530, Build:Verizon/heroqltevzw/heroqltevzw:7.0/NRD90M/G930VVRU4BQA2:user/release-keys

Samsung SM G925F (Galaxy S6 Edge), OS:6.0.1, CPU:arm64-v8a, GPU:Mali-T760, Build:samsung/zeroltexx/zerolte:6.0.1/MMB29K/G925FXXU5DQA7:user/release-keys

Samsung GT N7100 (Galaxy Note 2), OS:4.4.2, CPU:armeabi-v7a, GPU:Mali-400 MP, Build:samsung/t03gxx/t03g:4.4.2/KOT49H/N7100XXSFQA1:user/release-keys

Notes: If Multithreaded rendering is turned on everything works as expected. NatCam Plugin is used.