Grapqhl query to get board list issues is slow

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Reproducer:

• go to Portfolio Management and certify board
• open web developer console in your browser
• select "group by: Epic" (supposing swimlanes are still enabled for gitlab-org)
• in network tab you can see 2 graphql queries - first one (getting board epics) is served in a reasonable time, the second request takes about 30s.

The second request loads board list issues, here is profiler output of the query: out.profile. From a brief check there might be couple of problems:

• the query to get paginated issues is slow (about 7s)
• there is an N+1 query issue (2300 SQL queries are done during the request) - it would be good to try to eliminate some of them if possible

Related to #229143 (closed)

UPDATE: originally this issue aimed optimization of loading issues when epic swimlanes are loaded (loading of swimlanes used to perform well), but now the major/primary bottleneck is slow query when loading swimlanes itself. So primary focus should be optimizing loading of swimlanes (this may be improved by removing expensive ordering from the query - #243606 (comment 680609188))