Gatsby V3 + Material UI v5 slow develop rebuild time

[VojtechKycelt]

Hello,
In my multi-page web project I have been using Material UI v4 with Gatsby v3, in development mode everything worked just fine, but when I built project, some styles were not loading correctly and i couldn't find solution so I upgraded Material UI to v5. Now develop and build are exactly the same, it solved my problems with styles and built project works perfectly, but develop mode is really slow. When I work in IntelliJ IDEA it is lagging and re-build time is very slow about 40-60sec (2 sec before the migration).
My project does not have any queries and it has only around 10 pages, so it is quiet small and should be fast.
Does anyone know what could cause these problems?
And do you know any solution?
Or should I try re-do project and not use Material ui, Gatsby or both?