Hi Rudolf,
We can't reproduce this behavior on any of our working and testing machines. As far as I know (and I test this thoroughly), there are no memory leaks in the current version of PixInsight with the current set of official modules.
I have to say that our testing machines have at least 16 GB of RAM (most of them have 32 GB, and my main workstation has 64 GB). Depending on the sizes of your images, the amount of images you open, how you process them, and what applications and utilities (including background processes and services) you are using along with PixInsight, 8 GB may not be enough to achieve an optimal user experience. This is particularly true on Windows. System resources are managed much better on macOS, and even better on Linux and FreeBSD.
Especially, opening file selector boxes takes 10-30 s ...
This does not happen under normal working conditions, unless you are running out of resources severely. Note that native file open dialog boxes are not created or managed by PixInsight on Windows; they are managed exclusively by your operating system.