additional recommendations

[carl-alberto]

Issues from theme review theme

The menu is horizontal with no spacing between items.
The sidebar is always at the bottom of the content, not on the side.
The sidebar is white, not gray.
The layout and colors are different in Customizer (more like screenshot) and front end.
The fallback menu is not responding to hover to show the dropdowns.
The dropdown submenus do not have a background color, so I can't read them over the other text.
The mobile menu is vertical, but still responds to hover to dropdown the submenus. This is difficult to use with a mouse, and almost impossible to use with a finger.
Comments are showing the ordered list number, which is confusing for paged and nested comments. Try using CSS to hide the number or CSS counters to make it more logical.
is quite small,

is tiny. How is this a heading?

The
tag is preserving whitespace (Yay!), but it uses a proportional font, so that whitespace is variable. It would be better to use a monospace font.
The tag is styled like blockquote (as a background color and no quotes). This works okay for a block, but not inline quotes, which is what is.
Remove rtl.css or put actual styles in it.
In security.php, the comment-reply script enqueue should be in a function hooked to wp_enqueue_scripts. The other two functions in there are really plugin territory.
In bootstrapfast_assets(), there is an unused variable which probably should be used instead of three calls to a function doing the same thing.
The function bootstrapfast_asset_head() is not really needed because jQuery is listed as a dependency for the navigation script. However, if you want to tell your users that the theme always loads jQuery for them, you could leave it (but be sure to document it!)
The name of function bootstrapfast_theme_prefix_setup() doesn't really say what it does, and the 'theme_prefix' part was supposed to be replaced by your theme prefix.
Instead of making other functions in custom-functions.php, can you just put the code in the one place called? Or call the file custom-logo or something? Remember, if you are going for speed, fewer files is faster.
The print preview for a post with comments is showing me only the first comment and then a blank page.
You might want to hide the menu and widget area for print styles.