Hello, AK.
We have created the issue for this problem in our bug-tracker:
https://ugene.unipro.ru/tracker/browse/UGENE-1940We will try to investigate and fix the problem of this slowness, and the solution will be available in the upcoming UGENE 1.12.2 release.
But there are two temporary solutions.
The first one is to switch the style of UGENE graphic interface.
1) Click the menu Settings -> Preferences.
2) In the first tab "General" find the combo box "Appearance".
3) Choose the "Cleanlook" item instead of "GTK+".
The second temporary solution is to download and use the portable version of UGENE from our site (but not from Ubuntu repository):
http://ugene.unipro.ru/download.htmlAs I can see, this version does not have the problem with open file dialogs.
Could you try it, please, and tell us the result?