In KDE4 (Kubuntu 14.04) there was a trash directory per file system (on /home it was somewhere under ~, on other filesystems it was (from memory) a /.trash/{id}/. The benefit was that trashing files was quick since they were just moved to that directory.
In KDE5 (Kubuntu 16.04) I only have one trash directory in /home, and deleting things on the second disk is slow (and induces wear in the SSD). Is there a way to configure trash to work as in KDE4?
Specifications and Standards
The KDE (*1) is using Freedesktop specifications: https://www.freedesktop.org/wiki/Specifications/ . In this case - Desktop Trash Can Specification: https://www.freedesktop.org/wiki/Specifications/trash-spec/
By the 'Trash directories' (short):
A system can have one or more trash directories.
An administrator can create an $topdir/.Trash directory. If an $topdir/.Trash directory is absent, an $topdir/.Trash-$uid directory is to be used as the user's trash directory for this device/partition. $uid is the user's numeric identifier.
When trashing a file, if this directory does not exist for the current user, the implementation MUST immediately create it, without any warnings or delays for the user.
If both (1) and (2) fail (that is, no $topdir/.Trash directory exists, and an attempt to create $topdir/.Trash-$uid fails), the implementation MUST either trash the file into the user's “home trash” or refuse to trash it.
At here, with the Dolphin file manager, 'trashing' the 'TrashTest.txt' file:
The /.Trash-$uid is created.
Does your user and the KDE session have the right to create the needed /.Trash-$uid ?
*1) There is no KDE5. There are KDE Frameworks 5 (KF5), Plasma 5 and the KDE Applications. The KF5 and the plasma version can be looked from the KInfocenter: https://www.kde.org/applications/system/kinfocenter/ . The KDE Applications version can be looked from the application help page.