Lag spike issues with Radeon RX 550

Sounds like a Thermal Throttling issue.
Please check your CPU and GPU temperatures.
If those are too high (ie. around 90°C or higher), then it's very likely that.

 

Usually Thermal Throttling is not as specific to only affect VRAM or specific part. I'd take for good the temperatures you are getting.
But I would also double-check the Clock Speed of both the CPU and GPU when the slow-down happens, to be sure.
(During Thermal Throttling, clock speeds are dropped to the minimum, to allow the hardware to slow down and cool. Check if you see any significant clock speed drop)

 

Ok, then I'd suggest the usual list of things to check:

• If you use Windows 10, check that you have 'Game Mode' disabled
• If you have any overclock, it's suggested to revert them
• Check if the issue occurs in Safe Mode too (Launcher > Support Tools) - Especially if you use mods
• Provide 'Performance Graphics' before and during the issue*

*To get performance graphs, go to Options > User Interface and turn on the 'Advanced Features' settings.
In the side-bar menu, you should now see a 'Performance' button. Click it, and resize the window so that its content is clearly readable.
Wait for the graph to fill, take a screenshot, and post here.

 

Yep, seems your GPU to be causing that.

I'm don't have much experiences with AMD GPUs, but you should check the various features they have, such as 'Radeon Chill' and 'Radeon Wattman'.
Those may be doing weird things..