tomee just adds some libs to tomcat. that means you can also use a different version of tomcat easily - so it's easy to get tweaks and bugfixes of the latest version of tomcat also for tomee and you can use the std. tomcat plugin for your ide. however, that also means you (can) get some overhead - currently the demo apps shipped with tomcat cause such an overhead during the server startup. just by removing the demo web-apps you can reduce the startup time of tomcat itself by about 50 percent (here from ~1.4 seconds to ~0.7 seconds). for tomee you can do the same, but you have to keep a webapp called "tomee" (it's needed to use remote-ejbs - so it isn't a demo). if you don't need remote-ejbs, you can remove all webapps shipped by tomee to improve the startup performance even further.
the following chart shows the differences of the mentioned constellations. the value for jboss as7 is just a reference value.
the following chart shows the overhead introduced by tomee (compared to a standalone tomcat).
all those values depend on the machine, operating-system,... - in addition the values are rounded and should just give a basic impression about the differences (please don't see it e.g. as a tomee vs as7 comparison).
there are 2 values printed during the tomcat/tomee startup. the first one is listed as "Initialization processed in ..." and the second one as "Server startup in ...". the values in the charts always show the sum of both ( "Server startup in ..." sounds like the sum but it is not the sum of the full startup process).
as7 is also based on tomcat but the integration is way deeper and therefore it isn't given that the startup of tomcat itself takes the same time as with tomcat standalone or tomee (that's the reason why there is no separation for as7 in the second chart).