Problem:

Hidding/revealing content runs into a timout because of the subtree search reindex inside vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/Search/Common/Slot/HideContent.php.
The subtree contains more than 1000 content objects ..

Kernel version:

$ composer info ezsystems/ezpublish-kernel | grep versions
versions : * v7.5.3

Possible solution: 

Instead of reindex the subtree directly inside the signal slot, spawn an async process that accomplish the reindex by using the general reindex command (ezplatform:reindex --subtree=<locationId>).
This package could be used for spwaning async processes: https://github.com/spatie/async