Hello, 

have you change parameter of your server since your update ?
what'is your browser ? 
have you refresh your cache ? 

No changes on the server.
I use chrome usually but I tested Firefox and Edge, same troubles. refresh of cache, cron didn't change anything

Which version did you migrate from ?

Please open browser console.
Refresh some pages. You'll see few trace lines.
Please post some traces here.

Dear Babynus,

Thank you to help me.
I updated from the previous version. I'm always updating it when I get the message.

Here is the text in the console : 
to download it : here
in text : 
dojo.js?version=V9.1.3.0285:2 [Deprecation] Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/.
req.getText @ dojo.js?version=V9.1.3.0285:2
_33 @ dojo.js?version=V9.1.3.0285:2
_b @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
_38 @ dojo.js?version=V9.1.3.0285:2
_7c @ dojo.js?version=V9.1.3.0285:2
_17 @ dojo.js?version=V9.1.3.0285:2
_8c @ dojo.js?version=V9.1.3.0285:2
load @ dojo.js?version=V9.1.3.0285:2
_ed @ dojo.js?version=V9.1.3.0285:2
_33 @ dojo.js?version=V9.1.3.0285:2
_b @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
_38 @ dojo.js?version=V9.1.3.0285:2
_7c @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
_b @ dojo.js?version=V9.1.3.0285:2
_80 @ dojo.js?version=V9.1.3.0285:2
_f6 @ dojo.js?version=V9.1.3.0285:2
_33 @ dojo.js?version=V9.1.3.0285:2
_b @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
_38 @ dojo.js?version=V9.1.3.0285:2
_7c @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
_b @ dojo.js?version=V9.1.3.0285:2
_80 @ dojo.js?version=V9.1.3.0285:2
_f6 @ dojo.js?version=V9.1.3.0285:2
_33 @ dojo.js?version=V9.1.3.0285:2
_b @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
_38 @ dojo.js?version=V9.1.3.0285:2
_7c @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
_b @ dojo.js?version=V9.1.3.0285:2
_80 @ dojo.js?version=V9.1.3.0285:2
_f6 @ dojo.js?version=V9.1.3.0285:2
_33 @ dojo.js?version=V9.1.3.0285:2
_b @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
_38 @ dojo.js?version=V9.1.3.0285:2
_7c @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
_b @ dojo.js?version=V9.1.3.0285:2
_80 @ dojo.js?version=V9.1.3.0285:2
_f6 @ dojo.js?version=V9.1.3.0285:2
_33 @ dojo.js?version=V9.1.3.0285:2
_b @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
_38 @ dojo.js?version=V9.1.3.0285:2
_7c @ dojo.js?version=V9.1.3.0285:2
_17 @ dojo.js?version=V9.1.3.0285:2
req @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
(anonymous) @ dojo.js?version=V9.1.3.0285:2
Show 25 more frames
projeqtor.js?version=V9.1.3.0285:180 error saving data to session param=allColorsDynamicCss, value={"key":"--color-reference","value":"*1814ff"},{"key":"--color-darker","value":"*303050"},{"key":"--color-dark","value":"*4d4d80"},{"key":"--color-medium","value":"*6966ff"},{"key":"--color-light","value":"*cdccff"},{"key":"--color-lighter","value":"*e6e5ff"},{"key":"--color-text","value":"*656565"},{"key":"--color-white","value":"*ffffff"},{"key":"--color-secondary","value":"*e97b2c"},{"key":"--color-darker-secondary","value":"*503d30"},{"key":"--color-dark-secondary","value":"*80624d"},{"key":"--color-medium-secondary","value":"*f1a874"},{"key":"--color-light-secondary","value":"*fae2d1"},{"key":"--color-lighter-secondary","value":"*fdf1e8"},{"key":"--color-background","value":"*ffffff"},{"key":"--color-toolbar","value":"*1814ff"},{"key":"--color-toolbar-text","value":"*ffffff"},{"key":"--color-toolbar-invert","value":"1"},{"key":"--color-toolbar-invert-reverse","value":"0"},{"key":"--color-menu","value":"*4d4d80"},{"key":"--color-list-header","value":"*ffffff"},{"key":"--color-list-header-text","value":"*4d4d80"},{"key":"--color-grid-header-bg","value":"*ffffff"},{"key":"--color-grid-header-text","value":"*4d4d80"},{"key":"--color-grid-selected-bg","value":"*e97b2c"},{"key":"--color-grid-selected-text","value":"*ffffff"},{"key":"--color-detail-header","value":"*ffffff"},{"key":"--color-detail-header-text","value":"*4d4d80"},{"key":"--color-detail-header-border","value":"*cdccff"},{"key":"--color-section-title-text","value":"*4d4d80"},{"key":"--color-section-title-border","value":"*4d4d80"},{"key":"--color-table-header","value":"*cdccff"},{"key":"--color-button-background","value":"*e6e5ff"},{"key":"--color-button-text","value":"*4d4d80"},{"key":"--image-hue-rotate","value":"250.91666666666666deg"},{"key":"--image-hue-rotate-reverse","value":"-250.91666666666666deg"},{"key":"--image-saturate","value":"50%"},{"key":"--image-brightness","value":"80%"},{"key":"--image-hue-rotate-selected","value":"52.91666666666667deg"},{"key":"--image-saturate-selected","value":"80%"},{"key":"--image-brightness-selected","value":"200%"},{"key":"--image-hue-rotate-selected-from-ref","value":"-198deg"}, saveUserParameter=false
dojo.js?version=V9.1.3.0285:2 _4c0 {message: "Unable to load ../tool/saveDataToSession.php?idDat…selected-from-ref","value":"-198deg"} status: 504", response: {…}, status: 504, responseText: "…stream server or application.

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I think you have an issue on your network or on your server.

You have many errors : Unable to load ../tool/saveDataToSession.php
With error status: 504, which is a gateway timeout.

The "saveDataToSession.php" is a very quick process : it saves data to the session.
There is no DB update nor complex computing.
This may come from a blocking network device (firewall, router, proxy) or issue on server (for instance blocked drive that cannot serialize/unserialize session data)

Thanks for the analyse.
I didn't changed anything on the server. It's the OVH hosting of my website.
And many colleagues experience the same problem (they have other connexions as they are all in homeworking)
How to go forward with this problem. Is it possible to downgrade?

You can downgrade, but you'll get many error when upgrading .
But I'm not sure this will change anything.
The request recorded as long in the log are very (yes very) quick ones.
So issue may come from elsewhere.

 It's the OVH hosting of my website.

Any possibility to face network issue after recent issues on OVH ?

Did you try to copy your instance on another hosting (or local) system to check performance ?

I copy it locally and it went very fast.
After checking, it was the version of php that wasn't the latest.
Upgrading to 7.45 increased speed and the errors dissapeard