I can confirm that I have also run into this issue since purchase of my card. I am using a MSI Mech OC 5700 XT. I was previously running an ASUS Strix R9 380 with none of the issues outlined below and would consistently get >150fps in LoL at maximum details.
Same as above, I have never exceeded the 144fps needed to match the refresh rate of my monitor since purchase of this card. Frequently throughout the game (LoL), somewhat randomly, there will be a 2.5s pause (consistent amount of time each pause) in the screen - the screen is frozen and nothing works. Whilst I am playing I also monitor the performance of the card in AMD's Adrenaline Software on my secondary display - during these pauses my GPU utilisation and clock speed go to 0% and then recover. My card is not overheating running at a maximum of 65 degrees as I have an open case with good airflow, and my PSU wattage is adequate. This issue has occurred much more rarely in other games and during general use, e.g. YouTube, but nowhere near to the same degree.
A new issue has now also arisen where randomly, LoL will just tab out to the desktop and you will have to click on the minimised client to re-enter the game.
My software is completely update (version 20.1.3) and I am still running into this issue.
I have also noticed, similar to what is outlined in this post here (5700 XT Brief White Noise upon Login to Windows ) is white noise on my secondary display when logging into windows.
Very strange, and VERY disappointing, given this was a nearly $700 up(down)grade here in Australia. My R9 380 suffered none of these issues. I have sent a warranty claim but it seems this is a more systemic than card specific issue.
