I live relatively far from the server and my ping varies from as low as 65 to as high as 120, with occasional spikes when rezoning and in PvP areas with a very high concentration of players. The latter are clearly server side problems.

I also experience stutter and reduced frame rate after extended periods of play, without any ping spikes, which are corrected trough client restart. I've tracked those mostly to memory allocation, since the process grows to over 4 GB memory occupation before it starts to occur - I have 8 GB but other processes use that as well. The client fails to shed old data in a timely fashion and thus occupies a lot of system memory up to the point it also starts using the swap file; that's when the stutter stats since the disk is much slower than the memory. Before I go in PvP or trials I always restart the client to prevent this happening in the first 1.5-2h of play.

This client side problem may indeed become worse as more DLCs are released, since more data is loaded, but can probably be corrected to some extent by upgrading system memory so the game doesn't start unloading unto the swap file. Also older GPUs may have problems rendering new textures from DLC areas, since they are clearly more polished than the vanilla zones and that will lead to lower frame rates, even with good connection, as well as the occasional "doughy" appearance and other rendering errors. But that doesn't mean the DLCs are "killing the game". The same things have happened for years when trying to run new games on existing, outdated hardware. I'm confident that they will streamline the code somewhat when they cut support for some legacy systems. For example they will cut 32b support in 2018Q1.