Would be interesting to know how many jobs, nodes, (folders?), views you have.
There are a few performance issues affecting large Jenkins instances, e.g.
Then there's the possibility that you have a view configuration requiring a lot of loading, e.g. the Projects Statistics Plugin (loading all jobs), or the 'Weather' column in list views.
It would be very helpful if you could post excerpts of the /threadDump page taken while a page is loading – the responsible thread is called Handling GET /....
Without this information, it's not possible to further investigate this.