Hi,
5.2.0 rc3 is in experimental. You can already try. (only a rc4 is planned until the final, see https://wiki.documentfoundation.org/ReleasePlan/5.2)
After a quick test from experimental (1:5.2.0~rc3-1). Without any libreoffice-gtk? = slow with libreoffice-gtk2 = slow with libreoffice-gtk3 = normal (ie. almost instant). I will be sticking with the gtk3 version. Regards, Andrew