Ok, so maybe this isn't abnormal on my little PC...
Maybe Typora? I've heard a lot about it and it's mentioned in the doc.
It's reassuring to know it should be quick, even with all my notes 
When I wrote this message:
I didn't have any plugins installed.
Now I just have one plugin: note tabs.
I just did it, but this obviously works better. I notice that it takes around 3s to display the contents of a notebook when I click on it, and when I click on the different notes in a notebook it takes around 1s, which is pretty good!
Here are the lines that appear when I click on notebooks, which take 2-3 seconds to display their contents (list of notes):
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:36 useHtmlLoader: isReady true
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:40 useHtmlLoader: setHtml a929c28f6d2f9501630433ab3ad1aeb4
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:36 useHtmlLoader: isReady true
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:40 useHtmlLoader: setHtml f2a9f3762b4ddea1a5d6c1abd874fa08
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:19 useHtmlLoader: message {target: 'UserWebview', message: 'htmlIsSet', hash: 'a929c28f6d2f9501630433ab3ad1aeb4'}
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:19 useHtmlLoader: message {target: 'UserWebview', message: 'htmlIsSet', hash: 'f2a9f3762b4ddea1a5d6c1abd874fa08'}
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\gui\NoteEditor\utils\useMessageHandler.js:27 Got ipc-message: noteRenderComplete undefined
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:36 useHtmlLoader: isReady true
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:40 useHtmlLoader: setHtml 7c7ae59bf1fca1f2781184bbaa23a9d2
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\gui\NoteEditor\utils\useMessageHandler.js:27 Got ipc-message: noteRenderComplete undefined
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:19 useHtmlLoader: message {target: 'UserWebview', message: 'htmlIsSet', hash: '7c7ae59bf1fca1f2781184bbaa23a9d2'}
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:36 useHtmlLoader: isReady true
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:40 useHtmlLoader: setHtml 2dd013e278b956f09634c3878fdd7e11
C:\Users\nonob\AppData\Local\Programs\Joplin\resources\app.asar\services\plugins\hooks\useHtmlLoader.js:19 useHtmlLoader: message {target: 'UserWebview', message: 'htmlIsSet', hash: '2dd013e278b956f09634c3878fdd7e11'}
Here is the information copied from Joplin, but if the slowness becomes significant again, I will create a support post:
Joplin 2.14.20 (prod, win32)
ID client : 773c93d7dee04e1289cb896f235df6a3
Version de Synchro : 3
Version du profil : 46
Trousseau supporté : Oui
Révision : cfd98e3
Backup: 1.4.0
Note Tabs: 1.4.0
Thanks 