@gonzaloriestra
I’m on 3.94.3 and I tried again with SHOPIFY_CLI_DISABLE_IMPORT_SCANNING=1 and it does the same, Sync-reads the files for a long time.
shopify app dev is fast !
I tried with 4.1.0 but it degraded shopify app dev to being slow again, it does the Sync-read thing too
I investigated a bit further and created another issue, this is quite critical and blocking our migration path to web components 
Thanks for taking a look!