fix/performance fixes

[mjaakko]

Short description

Refactoring and fixes to some performance problems

DoD

I have:

• [x] added actual code.
• [x] produced clean code that passes ESLint.
• [x] code that actually does what it should.
• [ ] documented the code or added documentation to the wiki.
• [ ] added or modified test(s).
• [ ] test(s) that actually pass.
• [ ] works in trunk branch

[hisahi]

I can't notice any crashes with a quick test of StudentTable. Are there any other components with major changes?

[hisahi]

I'll separate this from #1084

[gotonode]

Will this make it before we say goodbye to Labtool?

[gotonode]

Should we just leave this open?