Hi Glycine and welcome to the forums.
On the issue of speed, there is a noticeable difference in
the time X5 takes to load a project. Even from X3 to X4 I remember
a difference. This is due to a number of factors not least the
underlying Access database. It can also be down to your project's
makeup (e.g. size, images, links, etc.). There may be little you
can do apart from perhaps break them down into smaller projects and
merge them together in a master project. Some of us have been known
to open a project, make a coffee and read the morning post before
starting work. OK maybe I jest but only a little bit
.
On importing the X3 projects, I'm a little less sure. I
haven't used X3 for years and never the X5 Pro version. However if
I remember correctly, when you go to open a project, there should
be an option to select a RH project created with X3. The file
extention of X3 projects is different to X5 so you'll need to
select the correct file type. Once you select the X3 project file,
the conversion takes place. Make sure to take backups and
thoroughly test all aspects before going live as there a number of
new features between these two versions.