I've been testing version 1.2.3 in both Windows 10 versions (PC and Mobile). PC has a top of the line specs (Intel Core i7 4790K, NVidia GTX 1080i GPU, 16 GB Ram, etc...) and the mobile too (Microsoft Lumia 950XL). It's not a performance problem with any of the devices, it performed flawlessly until version 1.2.3. I've tested with the wheat problem and sure, it drops performance from about 40 fps to less than 5 fps. Also being in villages at night, same problem happens and then again frame rate drops abruptly. Tested in Nether and End worlds, and it got back some frames, yet, I've noticed that 40 fps performance is only from version 1.2.3, before I got nearly 60 fps in PC and 50 in Mobile. Also I've noticed a cycling lag that happens every 5 seconds, doesn't matter what you're doing, if you are near or far away from any passive mobs. And finally, I've found also a severe frame rate drop when any aggressive mobs are present, either night or day, it just drops fps to less than 10. I've cited the other reports in which some people have the same issues. Hope this is fixed soon. Thanks.

MCPE-28028
MCPE-28140
MCPE-28007
MCCE-6232
MCPE-28022
MCPE-27967