load in less than 2s

loading in 8+s

I have a treeview with almost 1000 nodes representing NACE rev2 and it loads in 400ms simple or in 8.4s if it has checkboxes, which is a lot. I'm interested in whether this time can be improved because other trees with checkboxes are much faster. I don't know how they are managed internally but I know that an array of 1000 checkboxes is much faster than 1000 separate checkboxes. I'm asking for advice on how to load faster because 8 seconds is not acceptable. NACE Rev2.1 has around 1200 nodes and I'm scared of how long it will take. Take a look: https://listafirme.ro/firme-industrii.asp