I can confirm this issue. It's the same here running two instances of foreman (1.5.1 and 1.5.2) and the page loads slow for all "older" hosts (i.e. the ones with many reports).

Same problem here with foreman 1.6.2. It is not very very slow but I have to wait at least 11s.

Processing by HostsController#show as HTML
Parameters: {"id"=>"hostname.domain"}
Rendered hosts/_overview.html.erb (9.6ms)
Rendered hosts/show.html.erb within layouts/application (11962.8ms)
Rendered home/_user_dropdown.html.erb (1.7ms)
Read fragment views/tabs_and_title_records-2 0.1ms
Rendered home/_topbar.html.erb (3.4ms)
Rendered layouts/base.html.erb (116.9ms)
Completed 200 OK in 12120.9ms (Views: 12071.6ms | ActiveRecord: 14.0ms)

Is there something we can do in our server to reduce this time ?

If someone can't update to 1.7 right now, deleting reports might be a workaround.

http://theforeman.org/manuals/1.6/index.html#3.5.4PuppetReports

Expire reports automatically

You will probably want to delete your reports after some time to limit database growth. To do so, you can set a cronjob:

Available conditions:

days => number of days to keep reports (defaults to 7)
status => status of the report (defaults to 0 --> "reports with no errors")
Example:

Expires all reports regardless of their status
foreman-rake reports:expire days=7
expires all non interesting reports after one day
foreman-rake reports:expire days=1 status=0

Without the reports the node-page takes 319.7ms instead of 22215.3ms