Investigate slowness with UKBB

[melissachang]

256 improves experience of most data explorers (1000 Genomes, NHS). However, it makes UKBB worse. For UKBB facets API call is slow, and it's strange that it takes a while for checkmark to appear after click.

As a quick fix, I reverted #256 in https://biobank-explorer.appspot.com/ I would like to investigate ways to fix the underlying slowness, so we can reenable #256 for UKBB.

• Would react-virtual help? (I haven't looked at network tab yet.)
• For NHS, it helped it not send so many facet values over. Originally, BMI data was a string and every single participant's BMI was sent as a facet value. Once BMI became a number, only buckets (eg 10-20) were sent, and the UI became much snappier. We could delete some facets that have many values, like u41200_0_0.
• Play around with data node machine type/memory/disk etc
• Elasticsearch doc Tune for search speed

[melissachang]

I removed a bunch of facets that have many facet values (eg u41202_0_0). Initial page load went from 11s to 3s. The UI is also snappier -- the problem described above is much better. This is good enough for now.