MM5 skins are more powerfull than MM4's skins and can change also the behaviour (not only look) -- so all js/css/html5 code needs to be reloaded and thus MM5 restarted. (i.e. it is not just about loading several PNGs like in MM4)

There is the warning that MM5 will reload: https://www.dropbox.com/s/68o0ph8dyjyql7v/screenshot%202021-02-01%2019.09.12.png?dl=0

As for taking this long time: This depends whether you are using debug build (where Eureka seems to slow down closing for several seconds), using release build makes the restart much faster.

EDIT: re your suggestion: it's already tracked as https://www.ventismedia.com/mantis/view.php?id=17382

A MM4 skin change took a fraction of a second.
A MM4 skin change happens like this:
[list]there is a dbox where you have to verify your request[/list]
[list]then MM5 appears to close ... this does not happen in MM4[/list]
[list]and then nothing seems to happen [b]for a long time[/b] ... at least 6 seconds .... or 13 seconds if MM5 is busy in the background importing artwork[/list]
[list]and then the MM5 splash screen appears, which shows some sign of life[/list]
[list]and then, after another 6 seconds, MM5 reopens with the new skin[/list]

That unexpected delay of 13 seconds is a problem ... even the delay of 6 seconds is not good:
[list=1]
[*] the user is not warned that the delay is going to a long delay
[*] the user is not warned that MM5 will (appear to) close
[*] there is no life sign from MM5 during the delay
[*] the expectation from MM4 is that this is an instantaneous operation
[*] it is quite likely that the user will try and manually restart MM5 during this delay .... MM5 remains exposed to causing multiple concurrent invocations within a shutdown|restart operation
[/list]

[b]Suggestion:[/b] The confirmation dbox should warn that MM5 will temporarily close, and that there will be 10+ second delay before normal business resumes