Frame rate drops to 1Hz with Show Portraits enabled when Druid summons Poison Creeper.
Toggling Show Portraits to off, switching to Legacy Mode, using Unsummon or summoning a Carrion Vine or Solar Creeper brings the frame rate back up to an acceptable level.
Update:
It’s completely fixed now! All I had to do was uninstall Diablo 2 Resurrected, install the original Diablo 2 and play that instead; everything is now functioning as it was designed! Thank you so much, Blizzard for your continuous support, earnest dedication, and especially your attention to detail!