Well, I don't know you & I don't like to assume or speculate, so i ask questions. :)
Haven't experienced many such problems. Occasionally, while trying to run old scenes, which after revising worked OK.
Could be shaders or any other settings, basically anything... and an answer without a scene file to look into is everything but an answer. Even Corona by itself is a bit slower with all the advancements added. That is why optimizations and improvements are on the road map for 1.6 ;)
Finally when testing, i have material previews locked (since running it is hell of a workflow bugger) and render only via VFB as it is best for now.
I like testing & playing, just hate wasting time on the broken stuff. And the major stuff is still considered broken in my book. Waiting for RC4.
As i have written, not a specific BUG, but thanks for the notice.
That's all.
Stay well!