The clock speed throttles to 0.4ghz while charging. Tried everything. XTU, Bios settings, vantages. It still throttles. Pretty sure it needs a BIOS update to fix the problem. 

Same problem here, mine is i7-8550u. 0.4ghz when charging, can't do anything with it.

I also get a lot notifications of "not charging" or "slow charging" while it charges fine, my charger is the 45w one.

Also found a reddit thread on this 

https://www.reddit.com/r/thinkpad/comments/9h2vjc/underwhelming_experience_with_the_thinkpad_l380/

I have the exact same problem but mine is just an L380 not Yoga.

My processor is an i7 8550U.

That makes the laptop almost unusable when charging. 

I have also the problem with L380 Yoga with i7 8550U chip that it reduces cpu speed extremly if power cable is plugged into the notebook and it is charging. Its not possible to work with this properly. Can anybody help? Otherwise i have to send the notebook back to lenovo.

Suddenly and without updating any driver I start having this issue. The problem with my L380 Yoga happens only when connected to the docking station but not when using the USB C charger, so laptop is still somehow usable.

I updated everything (including bios) lenovo system update. Still same results and not clear if this is a software or hardware issue, laptop is only 2 months old. Another thing I've noticed is that this is something that doesn't happens inmediately the first time you boot up the PC, it needs a few minutes. Then the issue can be reproduced very fast.

While on docking station CPU multiplier gets stuck at 4x even if I try to force a task with high CPU consumption. Laptop becomes unusable.

Simply disconnecting the docking station immediately unlocks the multiplier and everything gets back to normal

I have tried another docking station and same results.

I had this behavior right from the beginning. It was unusable. What do i need a high speed i7 cpu if its not allowed to develop full speed and throttled to 0,8 ghz instead of 3.0 or 4.0 if the ac cable is plugged in. All latest drivers installed, bios updated, energy settings of windows to 100 percent on battery and ac. Support Hotline could not help. I sended it back to lenovo in the end.

I now sended back the L380 Yoga with "U" CPU and switched to a Thinkpad W541 i7 with "MQ" CPU for applications with high CPU usage and parallel a X201 Tablet for peninput or outdoor usage. No Problems with W541 with CPU throttling up to now. 50-60 CPU Usage in Apps where i had 100 Percent Usage with L380 Yoga.

My Yoga c940, brand new had it for a month started to get this issue. I've tried updating the bios, driver updates, everything. Nothing, works, it happens on or off the power cable and I can't seem to find the issue. Has there been any fix?