The following patch improves the performance when retrieving objects to render especially when there are many visible projects.

Index: app/helpers/my_helper.rb
===================================================================
--- app/helpers/my_helper.rb    (revision 17763)
+++ app/helpers/my_helper.rb    (working copy)
@@ -78,7 +78,7 @@
   def render_calendar_block(block, settings)
     calendar = Redmine::Helpers::Calendar.new(User.current.today, current_language, :week)
     calendar.events = Issue.visible.
-      where(:project_id => User.current.projects.pluck(:id)).
+      where(:project => User.current.projects).
       where("(start_date>=? and start_date<=?) or (due_date>=? and due_date<=?)", calendar.startdt, calendar.enddt, calendar.startdt, calendar.enddt).
       includes(:project, :tracker, :priority, :assigned_to).
       references(:project, :tracker, :priority, :assigned_to).
@@ -142,7 +142,7 @@

   def render_news_block(block, settings)
     news = News.visible.
-      where(:project_id => User.current.projects.pluck(:id)).
+      where(:project => User.current.projects).
       limit(10).
       includes(:project, :author).
       references(:project, :author).

require 'benchmark/ips'

include Redmine::Utils::DateCalculation

calendar = Redmine::Helpers::Calendar.new(User.current.today, :en, :week)

Benchmark.ips do |x|
  x.report('before') do
    Issue.visible.
      where(:project_id => Project.all.pluck(:id)).
      where("(start_date>=? and start_date<=?) or (due_date>=? and due_date<=?)", calendar.startdt, calendar.enddt, calendar.startdt, calendar.enddt).
      includes(:project, :tracker, :priority, :assigned_to).
      references(:project, :tracker, :priority, :assigned_to)
  end

  x.report('after') do
    Issue.visible.
      where(:project => Project.all).
      where("(start_date>=? and start_date<=?) or (due_date>=? and due_date<=?)", calendar.startdt, calendar.enddt, calendar.startdt, calendar.enddt).
      includes(:project, :tracker, :priority, :assigned_to).
      references(:project, :tracker, :priority, :assigned_to)
  end

  x.compare!
end

$ bin/rails r bench.rb
Warming up --------------------------------------
              before     5.000  i/100ms
               after    64.000  i/100ms
Calculating -------------------------------------
              before     68.797  (± 7.3%) i/s -    345.000  in   5.040461s
               after    652.797  (± 6.0%) i/s -      3.264k in   5.018461s

Comparison:
               after:      652.8 i/s
              before:       68.8 i/s - 9.49x  slower