It seems that from 18.06 LuCI interface has changed significantly with several improvements.
From what I could notice it consumes a bit of extra CPU (can't precise how significant yet) and also extra storage space. For this last part wanted to find out how big is the difference between the new LuCI and the previous one.
I am asking that because I have seen some older 32MB RAM/8MB Flash devices that although 18.06 compile and run fine they may struggle a bit when using the new LuCI.
Also I have seen people reporting the for some Tiny builds the new LuCI may consume extra space that could make some difference for other more essential packages for those cases.
So the question is if it's possible to have the older LuCI (from 17.01) in 18.06 builds and Snapshot so have a lighter interface for older router models or use less storage space for when Tiny builds are necessary but keep the newest features from 18.06 and Snapshot versions ?
Are you sure it is LUCI and not the rest?
The new kernel and applications are probably the "heavier" parts making the router slower. There are people reporting a lot slower transfer rate. It may also be because of driver optimized for the old kernel.
Do you see significant slow down on the devices you own?
Does it happen only when you install LUCI ?
Well from my first seen it looked LuCI was the one consuming extra CPU compared to the other, but you may be right and the newer kernel may be the responsible for that. Requires extra testing and use to find out.
Yes I have seen some slowdown one some older router (32MB RAM and 8MB Flash) I have tested with 18.06. As the router boots up it works fine, but as time passes and if you access LuCI and/or SSH for example it starts to get slow and slow (including increasing the latency significantly for packet forward) until the point it stops responding. In this case I had to revert back to a previous major version (with older Kernel and LuCi version).
So I am wondering that if for some older hardware but still with enough RAM and Flash even if you manage to build a 18.06 image it may be not very suitable to run and 17.01 will be the last stable version for them.