Unity 5.6 to 2017/2018 upgrade: frame drops

Hi,

This week, I did go back to an old (multiplayer) project after a few years. This is a big project (MiniStrike) and it is using the old network API (custom master server, facilitator, etc..).

So it's time to update my project from 5.6 to 2018.1 (latest version with old networking).

Unfortunately, I do have a problem with it on android. With Unity 5.6, I do get a constant 60 frame per seconds. But with 2018.1, I do have some major slowdowns in frame rate. Every seconds or less, the fps drops to ~40 which cause "major" slowdowns. I did spent a lot of time (~3 days) on this before asking some help here, without success. I did try other unity versions (2017, 2018) with the same results. The game is, I think, well optimized (batching, vertex lit rendering/shaders, no pixel lights, shadows...) and runs very fine on 5.6. I did also try to get more information's from the profiler, but couldn't find a problem. Disabling inputs, ui and meshes doesn't help neither (!?). I did also play with vsync with seemed a good idea, I could see that between versions it didn't react the same but it doesn't seems to be the problem.

So, I wonder if someone did have the same problem (if you can remember the time when 2017 was out..), or could give me some directions. For example, I wonder if nested coroutines could be a problem (I saw that the way coroutines are handled changed).

Many thanks in advance,
Cpas.