System:
Linux Mint 18 KDE Edition
Plasma 5.8.4
KDE Framework 5.28.0
Qt 5.6.1
Kernel 4.4.0-21-generic
OS type 64bit
Processors 4xAMD Athlon II X4 645
RAM 7.8 GiB

Hello.
When qBittorrent is in the notification area (in shown or hidden mode of tray entry does not matter), then plasmashell process uses from ~4% to ~18% of total CPU in spurts:

(this is the summary CPU usage, but peaks belong to plasmashell process).

When I disable "Show qBittorrent in notification area" in the Options of qBittorrent, then CPU peaks disappear, plasmashell uses about 4% of CPU:


I think cause is the speed information when mouse is over the qBittorrent entry in the notification area (tray) and popup with up/down speeds is shown: speeds is updated about each second and CPU peaks is about each second. But when mouse is not over the tray entry, CPU peaks continue to be, so I suppose that speeds is updated even when popup is not shown/visible...

Is it a bug of plasmashell or qbittorrent?

I don't know if it's a bug of the plasma desktop or not but being a qbittorrent user for a long time i can tell you that keeping the client minimized on tray does not help with mitigating unnecessary cpu usage. As you are in linux you can always run the client on one desktop and do other things on other desktop, disable the option that displays speed on title bar. It's all basic stuff, you need all of the resources to be used to downloading files not do anything else. Also simultaneously seeding and downloading a lot of number of torrents uses lot of resources. I discovered that while i was seeding like 50 torrents and downloading at a time.

ponpon wrote: disable the option that displays speed on title bar

No such option. And no speed info in the title bar of application window. I can only totally disable showing the qbittorrent in the notification area, but cannot disable only speed information in the popup window of the tray entry. Total tray entry disabling is uncomfortable — in this case qbittorrent is shown in the task bar. QBittorrent must be shown somewhere in order to comfortably working with it, so totally background process doesn't approach.

For note: "popup window" is the next it when mouse over the tray icon:

ponpon wrote:Also simultaneously seeding and downloading a lot of number of torrents uses lot of resources.

In the my system qBittorrent uses relatively few resources [<=2% CPU, <=220 MiB RAM] (as opposed to plasmashell when qbittorrent tray entry is enabled).