"show tables" slow with many tables compared to information_schema.tables

The query "show tables" on a database with many tables (>2M) is much slower than "select table_name from information_schema.tables where table_schema='database1'". I'm not sure if this is the same issue as MDEV-191.

Compare:

~ # time mysql -e "select table_name from information_schema.tables where table_schema='database1';" | wc -l
2498519

real 0m37.912s
user 0m1.048s
sys 0m0.132s

With:

~ # time mysql -e "use database1; show tables" | wc -l
2498575

real 2m16.279s
user 0m1.132s
sys 0m0.124s