The name of the proc is misleading, since it really executes the tcl/adp pair. Especially for /dotlrn the time depends on many factors, e.g. on what's portlets are on the start page, and how many memberships a user has. Another important spot are the master-templates (if you concerned about speed, try to avoid db-requests in the master templates).
On our production system, we have currently 763 users active, and i get for my dotlrn start page the following output
+0.1 ms: Served file /var/www/production/packages/dotlrn/www/index.adp with adp_parse_ad_conn_file - 1.7 ms
from the line you are looking at. We have essentially a 5.3.0 kernel with many local changes and optimizations (last time i counted, there were 15000 differences). This line is certainly no good starting point for optimizations.
My recommendation is to look at the most frequently used pages on your system (request-monitor/stat-details) and optimize the top 10%. For these pages, try to reduce the SQL queries (get rid of unneeded features, use caching) and optimize these.
